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, 27 insertions, 0 deletions

diff --git a/doc/COMPLETED.md b/doc/COMPLETED.md
index a4e6a94..a3c173d 100644
--- a/doc/COMPLETED.md
+++ b/doc/COMPLETED.md
@@ -2,6 +2,33 @@
 
 This file tracks recently completed tasks, organized by completion date.
 
+## Archive Index
+
+Detailed historical documents have been moved to `doc/archive/` for reference:
+
+**Analysis & Architecture:**
+- ANALYSIS_VARIABLE_TEMPO.md, ANALYSIS_VARIABLE_TEMPO_V2.md
+- AUDIO_LIFECYCLE_REFACTOR.md, AUDIO_TIMING_ARCHITECTURE.md
+- BUILD_OPTIMIZATION_PROPOSAL.md, BUILD_OPTIMIZATION_PROPOSAL_V2.md
+- PLATFORM_ANALYSIS.md, SHADER_PARAMETRIZATION_PLAN.md
+
+**Handoffs & Summaries:**
+- HANDOFF.md, HANDOFF_2026-02-04.md, HANDOFF_2026-02-08.md, HANDOFF_CLAUDE.md
+- HANDOFF_SCENE_LOADER.md, HANDOFF_SPECTRAL_EDITOR.md
+- PLATFORM_SIDE_QUEST_SUMMARY.md, TASKS_SUMMARY.md
+
+**Debug Reports:**
+- DEBUG_SHADOWS.md, PEAK_FIX_SUMMARY.md, PEAK_METER_DEBUG.md
+- SAMPLE_ACCURATE_TIMING_FIX.md, VISUAL_DEBUG.md
+
+**Testing & Reports:**
+- FINAL_STRIP_REPORT.md, GPU_EFFECTS_TEST_ANALYSIS.md
+
+**Planning Docs:**
+- PHASE2_COMPRESSION.md, SPEC_EDITOR.md, STRIPPING.md
+
+Use `read @doc/archive/FILENAME.md` to access archived documents.
+
 ## Recently Completed (February 8, 2026)
 
 - [x] **Shader Parametrization System (Task #73 Phase 0)** (February 8, 2026)