Update config assumptions doc - mark binary star issue as fixed

Document that barycentric orbit calculations have been implemented
and binary star systems now work correctly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/config_assumptions.md

@@ -26,11 +26,12 @@ The moon positions appear to use **absolute coordinates** matching their orbital
 
 ### **3. Binary Star Configuration Issues:**
 
-**Problem 1: Both stars have parent_index = -1**
+**~~Problem 1: Both stars have parent_index = -1~~ - FIXED**
 - StarA: `parent_index = -1`
 - StarB: `parent_index = -1`
-- This means both are treated as root bodies with zero velocity (config_loader.cpp:69)
-- They won't orbit each other!
+- ~~This means both are treated as root bodies with zero velocity (config_loader.cpp:69)~~
+- ~~They won't orbit each other!~~
+- **FIXED**: System now detects multiple root bodies, calculates barycenter, and sets appropriate orbital velocities (config_loader.cpp:64-146, bodies.cpp:101-124)
 
 **Problem 2: Planet positions**
 - PlanetA1 is at `(3.5e11, 0, 0)` while StarA is at `(3.0e11, 0, 0)` - only 50 million km apart