feat: Implement Hohmann Transfer tab (Phase 3)

Added complete Hohmann transfer calculation and UI:
- Spacecraft, Transfer Parent, Current Body, Target Body selectors
- Calculate Transfer button with real-time calculation
- Results panel showing dv1, dv2, transfer time, semi-major axis
- Create Both Burns / Create Burn 1 Only buttons
- Auto-generated maneuver names with format 'Hohmann Burn N (Source→Target)'
- Proper field initialization and validation

Fixed body selector logic with separate current_body_index field.

src/ui_renderer.cpp

@@ -494,6 +494,7 @@ void render_maneuver_create_tab(SimulationState* sim, UIState* ui_state, int x,
     Rectangle dir_label = {(float)x, (float)current_y, 80, 25};
     GuiLabel(dir_label, "Direction:");
 
+    // FIXME: we need to handle state differently here: see raygui controls_test_suite.c example
     if (ui_state->maneuver_dialog.craft_index >= 0 && ui_state->maneuver_dialog.craft_index < sim->craft_count) {
         Rectangle dir_combo = {(float)x + 85, (float)current_y, (float)(width - 95), 25};
         GuiDropdownBox(dir_combo, "Prograde;Retrograde;Normal;Antinormal;Radial In;Radial Out;Custom", &ui_state->maneuver_dialog.direction_active, false);
@@ -634,6 +635,208 @@ void render_orbital_preview(SimulationState* sim, UIState* ui_state, Rectangle b
     GuiLabel(status_bounds, status_text);
 }
 
+void render_maneuver_hohmann_tab(SimulationState* sim, UIState* ui_state, int x, int y, int width, int height) {
+    int current_y = y;
+
+    if (ui_state->maneuver_dialog.craft_index >= 0 &&
+        ui_state->maneuver_dialog.craft_index < sim->craft_count) {
+        Spacecraft* craft = &sim->spacecraft[ui_state->maneuver_dialog.craft_index];
+        if (craft->parent_index >= 0 && craft->parent_index < sim->body_count) {
+            ui_state->maneuver_dialog.current_body_index = craft->parent_index;
+        }
+    }
+
+    GuiLabel({(float)x, (float)current_y, (float)width, 25}, "Calculate optimal Hohmann transfer between orbits");
+    current_y += 30;
+
+    GuiLabel({(float)x, (float)current_y, 120, 25}, "Spacecraft:");
+    current_y += 35;
+
+    if (sim->craft_count > 0) {
+        int craft_buffer_size = sim->craft_count * 64 + 1;
+        char* craft_list = (char*)malloc(craft_buffer_size);
+        if (craft_list) {
+            craft_list[0] = '\0';
+            int offset = 0;
+            for (int i = 0; i < sim->craft_count; i++) {
+                if (i > 0) {
+                    offset += snprintf(craft_list + offset, craft_buffer_size - offset, ";");
+                }
+                offset += snprintf(craft_list + offset, craft_buffer_size - offset, "%s", sim->spacecraft[i].name);
+            }
+
+            GuiDropdownBox({(float)x + 120, (float)(current_y - 35), (float)(width - 130), 25},
+                          craft_list, &ui_state->maneuver_dialog.craft_index, false);
+            free(craft_list);
+        }
+    }
+    current_y += 35;
+
+    GuiLabel({(float)x, (float)current_y, 120, 25}, "Transfer Parent:");
+    current_y += 35;
+
+    if (sim->body_count > 0) {
+        int body_buffer_size = sim->body_count * 64 + 1;
+        char* body_list = (char*)malloc(body_buffer_size);
+        if (body_list) {
+            body_list[0] = '\0';
+            int offset = 0;
+            for (int i = 0; i < sim->body_count; i++) {
+                if (i > 0) {
+                    offset += snprintf(body_list + offset, body_buffer_size - offset, ";");
+                }
+                offset += snprintf(body_list + offset, body_buffer_size - offset, "%s", sim->bodies[i].name);
+            }
+
+            GuiDropdownBox({(float)x + 120, (float)(current_y - 35), (float)(width - 130), 25},
+                          body_list, &ui_state->maneuver_dialog.transfer_body_index, false);
+            free(body_list);
+        }
+    }
+    current_y += 35;
+
+    GuiLabel({(float)x, (float)current_y, 120, 25}, "Current Body:");
+    current_y += 35;
+
+    if (sim->body_count > 0) {
+        int body_buffer_size = sim->body_count * 64 + 1;
+        char* body_list = (char*)malloc(body_buffer_size);
+        if (body_list) {
+            body_list[0] = '\0';
+            int offset = 0;
+            for (int i = 0; i < sim->body_count; i++) {
+                if (i > 0) {
+                    offset += snprintf(body_list + offset, body_buffer_size - offset, ";");
+                }
+                offset += snprintf(body_list + offset, body_buffer_size - offset, "%s", sim->bodies[i].name);
+            }
+
+            GuiDropdownBox({(float)x + 120, (float)(current_y - 35), (float)(width - 130), 25},
+                          body_list, &ui_state->maneuver_dialog.current_body_index, false);
+            free(body_list);
+        }
+    }
+    current_y += 35;
+
+    GuiLabel({(float)x, (float)current_y, 120, 25}, "Target Body:");
+    current_y += 35;
+
+    if (sim->body_count > 0) {
+        int body_buffer_size = sim->body_count * 64 + 1;
+        char* body_list = (char*)malloc(body_buffer_size);
+        if (body_list) {
+            body_list[0] = '\0';
+            int offset = 0;
+            for (int i = 0; i < sim->body_count; i++) {
+                if (i > 0) {
+                    offset += snprintf(body_list + offset, body_buffer_size - offset, ";");
+                }
+                offset += snprintf(body_list + offset, body_buffer_size - offset, "%s", sim->bodies[i].name);
+            }
+
+            GuiDropdownBox({(float)x + 120, (float)(current_y - 35), (float)(width - 130), 25},
+                          body_list, &ui_state->maneuver_dialog.target_body_index, false);
+            free(body_list);
+        }
+    }
+    current_y += 35;
+
+    Rectangle calculate_btn = {(float)x, (float)current_y, (float)width, 30};
+    if (GuiButton(calculate_btn, "Calculate Transfer")) {
+        calculate_hohmann_for_dialog(sim, ui_state);
+    }
+    current_y += 40;
+
+    if (ui_state->maneuver_dialog.show_hohmann_preview) {
+        HohmannTransfer* ht = &ui_state->maneuver_dialog.last_calc;
+
+        char transfer_info[256];
+        double current_r = (ui_state->maneuver_dialog.current_body_index >= 0 &&
+                          ui_state->maneuver_dialog.current_body_index < sim->body_count) ?
+                          sim->bodies[ui_state->maneuver_dialog.current_body_index].orbit.semi_major_axis : 0.0;
+        double target_r = (ui_state->maneuver_dialog.target_body_index >= 0 &&
+                         ui_state->maneuver_dialog.target_body_index < sim->body_count) ?
+                         sim->bodies[ui_state->maneuver_dialog.target_body_index].orbit.semi_major_axis : 0.0;
+
+        snprintf(transfer_info, sizeof(transfer_info),
+                "Transfer semi-major axis: %.3e m",
+                (current_r + target_r) / 2.0);
+
+        Rectangle info_bounds = {(float)x + 8, (float)current_y, (float)width - 16, 25};
+        GuiLabel(info_bounds, transfer_info);
+        current_y += 25;
+
+        char time_info[128];
+        double days = ht->transfer_time / 86400.0;
+        snprintf(time_info, sizeof(time_info), "Transfer time: %.1f days", days);
+        Rectangle time_bounds = {(float)x + 8, (float)current_y, (float)width - 16, 25};
+        GuiLabel(time_bounds, time_info);
+        current_y += 30;
+
+        char burn1_info[256];
+        snprintf(burn1_info, sizeof(burn1_info),
+                "Burn 1 (at periapsis): Δv1 = %.2f km/s, Prograde, ν = 0.00 rad",
+                ht->dv1 / 1000.0);
+        Rectangle burn1_bounds = {(float)x + 8, (float)current_y, (float)width - 16, 25};
+        GuiLabel(burn1_bounds, burn1_info);
+        current_y += 25;
+
+        char burn2_info[256];
+        snprintf(burn2_info, sizeof(burn2_info),
+                "Burn 2 (at apoapsis): Δv2 = %.2f km/s, Prograde, ν = %.2f rad",
+                ht->dv2 / 1000.0, ht->true_anomaly_2);
+        Rectangle burn2_bounds = {(float)x + 8, (float)current_y, (float)width - 16, 25};
+        GuiLabel(burn2_bounds, burn2_info);
+        current_y += 30;
+
+        char total_info[128];
+        snprintf(total_info, sizeof(total_info), "Total Δv: %.2f km/s", (ht->dv1 + ht->dv2) / 1000.0);
+        Rectangle total_bounds = {(float)x + 8, (float)current_y, (float)width - 16, 25};
+        GuiLabel(total_bounds, total_info);
+        current_y += 30;
+
+        Rectangle both_btn = {(float)x, (float)current_y, (float)(width / 2 - 5), 30};
+        Rectangle burn1_btn = {(float)x + (float)(width / 2 + 5), (float)current_y, (float)(width / 2 - 5), 30};
+
+        if (GuiButton(both_btn, "Create Both Burns")) {
+            create_hohmann_burns(sim, ui_state, true);
+        }
+
+        if (GuiButton(burn1_btn, "Create Burn 1 Only")) {
+            create_hohmann_burns(sim, ui_state, false);
+        }
+    }
+}
+
+void calculate_hohmann_for_dialog(SimulationState* sim, UIState* ui_state) {
+    ui_state->maneuver_dialog.show_hohmann_preview = false;
+
+    if (ui_state->maneuver_dialog.transfer_body_index < 0 ||
+        ui_state->maneuver_dialog.transfer_body_index >= sim->body_count ||
+        ui_state->maneuver_dialog.target_body_index < 0 ||
+        ui_state->maneuver_dialog.target_body_index >= sim->body_count) {
+        return;
+    }
+
+    if (ui_state->maneuver_dialog.transfer_body_index == ui_state->maneuver_dialog.target_body_index) {
+        return;
+    }
+
+    CelestialBody* central_body = &sim->bodies[ui_state->maneuver_dialog.transfer_body_index];
+    CelestialBody* current_body = &sim->bodies[ui_state->maneuver_dialog.current_body_index];
+    CelestialBody* target_body = &sim->bodies[ui_state->maneuver_dialog.target_body_index];
+
+    double r1 = current_body->orbit.semi_major_axis;
+    double r2 = target_body->orbit.semi_major_axis;
+
+    if (r1 <= 0 || r2 <= 0) {
+        return;
+    }
+
+    ui_state->maneuver_dialog.last_calc = calculate_hohmann_transfer(r1, r2, central_body->mass);
+    ui_state->maneuver_dialog.show_hohmann_preview = true;
+}
+
 void render_maneuver_dialog(SimulationState* sim, RenderState* render_state, UIState* ui_state) {
     if (!ui_state->maneuver_dialog.open) {
         return;
@@ -686,6 +889,7 @@ void render_maneuver_dialog(SimulationState* sim, RenderState* render_state, UIS
     if (ui_state->maneuver_dialog.active_tab == MD_TAB_CREATE) {
         render_maneuver_create_tab(sim, ui_state, x + 8, y + 60, width - 16, height - 100);
     } else if (ui_state->maneuver_dialog.active_tab == MD_TAB_HOHMANN) {
+        render_maneuver_hohmann_tab(sim, ui_state, x + 8, y + 60, width - 16, height - 100);
     } else if (ui_state->maneuver_dialog.active_tab == MD_TAB_EDIT) {
     }
 }
@@ -839,3 +1043,63 @@ void update_maneuver_from_dialog(SimulationState* sim, UIState* ui_state) {
 
     ui_state->maneuver_dialog.open = false;
 }
+
+void create_hohmann_burns(SimulationState* sim, UIState* ui_state, bool create_both_burns) {
+    if (ui_state->maneuver_dialog.craft_index < 0 ||
+        ui_state->maneuver_dialog.craft_index >= sim->craft_count) {
+        return;
+    }
+
+    if (!ui_state->maneuver_dialog.show_hohmann_preview) {
+        return;
+    }
+
+    HohmannTransfer* ht = &ui_state->maneuver_dialog.last_calc;
+
+    const char* current_name = (ui_state->maneuver_dialog.transfer_body_index >= 0 &&
+                                ui_state->maneuver_dialog.transfer_body_index < sim->body_count) ?
+                               sim->bodies[ui_state->maneuver_dialog.transfer_body_index].name : "Unknown";
+    const char* target_name = (ui_state->maneuver_dialog.target_body_index >= 0 &&
+                              ui_state->maneuver_dialog.target_body_index < sim->body_count) ?
+                             sim->bodies[ui_state->maneuver_dialog.target_body_index].name : "Unknown";
+
+    char burn1_name[128];
+    snprintf(burn1_name, sizeof(burn1_name), "Hohmann Burn 1 (%s→%s)", current_name, target_name);
+
+    Maneuver burn1 = create_maneuver(
+        burn1_name,
+        ui_state->maneuver_dialog.craft_index,
+        BURN_PROGRADE,
+        ht->dv1,
+        TRIGGER_TRUE_ANOMALY,
+        0.0
+    );
+
+    int burn1_index = add_maneuver_to_simulation(sim, &burn1);
+
+    if (burn1_index < 0) {
+        return;
+    }
+
+    if (create_both_burns) {
+        char burn2_name[128];
+        snprintf(burn2_name, sizeof(burn2_name), "Hohmann Burn 2 (%s→%s)", current_name, target_name);
+
+        Maneuver burn2 = create_maneuver(
+            burn2_name,
+            ui_state->maneuver_dialog.craft_index,
+            BURN_PROGRADE,
+            ht->dv2,
+            TRIGGER_TRUE_ANOMALY,
+            ht->true_anomaly_2
+        );
+
+        int burn2_index = add_maneuver_to_simulation(sim, &burn2);
+
+        if (burn2_index < 0) {
+            return;
+        }
+    }
+
+    ui_state->maneuver_dialog.open = false;
+}

src/ui_renderer.h

@@ -26,6 +26,7 @@ struct ManeuverDialogState {
     int edit_maneuver_index;
 
     int target_body_index;
+    int current_body_index;
     int transfer_body_index;
     bool show_hohmann_preview;
     HohmannTransfer last_calc;
@@ -64,5 +65,8 @@ bool validate_maneuver_inputs(SimulationState* sim, UIState* ui_state);
 void generate_maneuver_name(SimulationState* sim, UIState* ui_state);
 void create_maneuver_from_dialog(SimulationState* sim, UIState* ui_state);
 void update_maneuver_from_dialog(SimulationState* sim, UIState* ui_state);
+void render_maneuver_hohmann_tab(SimulationState* sim, UIState* ui_state, int x, int y, int width, int height);
+void calculate_hohmann_for_dialog(SimulationState* sim, UIState* ui_state);
+void create_hohmann_burns(SimulationState* sim, UIState* ui_state, bool create_both_burns);
 
 #endif