docs: Archive historical documentation (26 files → doc/archive/)

Moved completed/historical docs to doc/archive/ for cleaner context:

Archived (26 files):
- Analysis docs: variable tempo, audio architecture, build optimization
- Handoff docs: 6 agent handoff documents
- Debug reports: shadows, peak meter, timing fixes
- Task summaries and planning docs

Kept (16 files):
- Essential: AI_RULES, HOWTO, CONTRIBUTING, CONTEXT_MAINTENANCE
- Active subsystems: 3D, ASSET_SYSTEM, TRACKER, SEQUENCE
- Current work: MASKING_SYSTEM, SPECTRAL_BRUSH_EDITOR

Updated COMPLETED.md with archive index for easy reference.

1 files changed, 40 insertions, 0 deletions

diff --git a/doc/archive/HANDOFF_SCENE_LOADER.md b/doc/archive/HANDOFF_SCENE_LOADER.md
new file mode 100644
index 0000000..b218d0b
--- /dev/null
+++ b/doc/archive/HANDOFF_SCENE_LOADER.md
@@ -0,0 +1,40 @@
+# Handoff: 3D Scene Pipeline (February 8, 2026)
+
+## Summary
+Implemented a complete pipeline for exporting 3D scenes from Blender and loading them at runtime.
+
+## Accomplishments
+
+### Task #18: 3D System Enhancements
+- **Blender Exporter**: Created `tools/blender_export.py` to export scenes to a binary format (`SCN1`).
+  - Exports objects, transforms, types, and mesh references.
+  - Handles string-based asset resolution.
+- **Asset System Update**: Updated `asset_packer` to generate `GetAssetIdByName` for runtime string lookup.
+- **Runtime Loader**: Implemented `SceneLoader` (`src/3d/scene_loader.h/cc`) to parse the binary scene format.
+- **Verification**: Added `test_scene_loader` to verify the pipeline.
+
+## Key Components
+
+### Binary Format (`doc/SCENE_FORMAT.md`)
+- Magic: `SCN1`
+- Supports Objects (Mesh, Primitives), Cameras, Lights.
+- Compact binary representation.
+
+### Runtime Integration
+- `SceneLoader::LoadScene(scene, data, size)` populates a `Scene` object.
+- Uses `GetAssetIdByName` to resolve mesh references (e.g. "MESH_CUBE" -> `ASSET_MESH_CUBE`).
+
+## Next Steps
+- Use the exporter in a real workflow (requires Blender).
+- Update `Renderer3D` or `MainSequence` to actually use `SceneLoader` for a level (e.g. `assets/final/level1.bin`).
+- Implement `Task #5: Spectral Brush Editor` (In Progress).
+
+## Files Modified
+- `tools/blender_export.py` (New)
+- `src/3d/scene_loader.h` (New)
+- `src/3d/scene_loader.cc` (New)
+- `src/tests/test_scene_loader.cc` (New)
+- `tools/asset_packer.cc` (Updated)
+- `src/util/asset_manager.h` (Updated)
+- `CMakeLists.txt` (Updated)
+- `doc/SCENE_FORMAT.md` (New)