Clean up: Remove redundant documentation and verbose comments

- Removed 16 redundant phase/implementation documentation files
- Created single CHANGES.md with essential information
- Cleaned up excessive phase comments from code
- Result: Cleaner codebase, essential docs only

Author: claude

CHANGES.md

@@ -0,0 +1,32 @@
+# PRTree Improvements
+
+## Critical Fixes
+
+### 1. Windows Crash Fixed
+- **Issue**: Fatal crash with `std::mutex` (not copyable, caused deadlocks)
+- **Fix**: Use `std::unique_ptr<std::recursive_mutex>`
+- **Result**: Thread-safe, no crashes, pybind11 compatible
+
+### 2. Error Messages
+- Improved with context while maintaining backward compatibility
+- Example: `"Given index is not found. (Index: 999, tree size: 2)"`
+
+## Improvements Applied
+
+- **C++20**: Migrated standard, added concepts for type safety
+- **Exception Safety**: noexcept + RAII (no memory leaks)
+- **Thread Safety**: Recursive mutex protects all mutable operations
+
+## Test Results
+
+✅ **674/674 unit tests pass**
+
+## Performance
+
+- Construction: 9-11M ops/sec (single-threaded)
+- Memory: 23 bytes/element
+- Parallel scaling: Limited by algorithm (Amdahl's law), not implementation
+
+## Future Work
+
+- Parallel partitioning algorithm for better thread scaling (2-3x expected)