remove old planning doc

docs/find_dominant_body_bug.md

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-# Find Dominant Body Bug: Mutual SOI Issue
-
-## Overview
-This documents the remaining issue with `find_dominant_body()` when two similar-mass bodies are positioned within each other's sphere of influence (SOI).
-
-**Date:** January 20, 2026
-**Status:** Failing Test (Test 4 from test_plan_invalid_parent_assignment.md)
-**Related:** `docs/test_plan_invalid_parent_assignment.md`
-
----
-
-## Test Case 4: Mutual SOI - Similar Mass Planets
-
-**Purpose:** Edge case: two Earth-like planets positioned within each other's SOI
-
-**Actual Result:** ❌ **STILL FAILS** (separate issue, not fixed by spacecraft validation)
-
-**Config:** `tests/configs/mutual_soi_close.toml`
-
-**Setup:**
-- PlanetA: mass = 5.972e24 kg, position = {1.496e11, 0, 0}
-- PlanetB: mass = 5.972e24 kg, position = {1.501e11, 0, 0}
-- Separation: 5e8 meters (500 million km)
-- Planet SOI: ~9.25e8 meters (925 million km)
-- **Both planets within each other's SOI**
-
-**Why This Fails:**
-- Both planets start with parent=0 (Sun)
-- Both are within each other's SOI
-- `find_dominant_body()` logic selects closest body within SOI
-- Result: PlanetA selects PlanetB as parent, PlanetB selects PlanetA as parent
-- Config validation passes (both are ≥ parent.radius + body.radius from Sun)
-
-**Key Assertions:**
-```cpp
-// Both should orbit Sun (not each other)
-REQUIRE(sim->bodies[PLANET_A_IDX].parent_index == SUN_IDX);
-REQUIRE(sim->bodies[PLANET_B_IDX].parent_index == SUN_IDX);
-
-// Planets should never have each other as parents
-for (int parent : history.planet_a_parents) {
-    REQUIRE(parent != PLANET_B_IDX);
-}
-for (int parent : history.planet_b_parents) {
-    REQUIRE(parent != PLANET_A_IDX);
-}
-```
-
-**Expected Behavior (Option A):**
-- Both planets should continue orbiting Sun
-- Neither should become other's parent
-- This requires future fix to `find_dominant_body()` logic (mass hierarchy check)
-
----
-
-## Future Work
-
-### Current Status
-Tests 1-3 from the parent assignment test suite are now passing. Test 4 continues to fail, documenting a separate mutual SOI issue.
-
-### Potential Fixes for Test 4
-1. Add mass hierarchy check to `find_dominant_body()`
-2. Prevent mutual SOI assignments for similar-mass bodies
-3. Detect and reject invalid configs at load time
-4. Implement proper N-body interaction or restricted 3-body solution
-
-### Enhanced Detection
-- SOI overlap detection at config load time
-- Automatic correction of invalid parent assignments
-- Validation warnings for edge cases
-
----
-
-## References
-- `src/simulation.cpp:64-107` - `find_dominant_body()` implementation
-- `tests/configs/mutual_soi_close.toml` - Test configuration for mutual SOI case
-- `tests/test_invalid_parent_assignment.cpp` - Test suite implementation