docs: update PROJECT_CONTEXT, TODO, COMPLETED for OLA-IDCT

- PROJECT_CONTEXT: audio section reflects OLA-IDCT (Hann, 50% overlap);
  test count 35->34; Next Up notes .spec regen needed
- TODO: remove stale MP3 sub-task (done), trim test TODOs, add .spec
  regen as Priority 3, update test count to 34/34
- COMPLETED: archive OLA-IDCT task with implementation summary

1 files changed, 9 insertions, 0 deletions

diff --git a/doc/COMPLETED.md b/doc/COMPLETED.md
index 0dba307..e37ddbf 100644
--- a/doc/COMPLETED.md
+++ b/doc/COMPLETED.md
@@ -29,6 +29,15 @@ Detailed historical documents have been moved to `doc/archive/` for reference:
 
 Use `read @doc/archive/FILENAME.md` to access archived documents.
 
+## Recently Completed (March 2, 2026)
+
+- [x] **OLA-IDCT Synthesis — click-free .spec decoding**
+    - **Goal**: Eliminate frame-boundary clicks in spectrogram→PCM synthesis.
+    - **Implementation**: Added v2 spectrogram format (`SPEC_VERSION_V2_OLA`). Synthesis uses Hann-windowed IDCT with 50% overlap-add (hop=256, overlap=256). Per-voice `overlap_buf[256]` accumulates the tail from the previous IDCT frame. V1 path (raw DCT-512) preserved for generated notes and old `.spec` files. Hann window precomputed at `synth_init()`. MP3 encoder switched from Hamming to Hann, now slides a 512-sample analysis window by 256 samples per frame (OLA analysis), emitting ~2× as many frames. `SpecHeader.version` field propagated through `Spectrogram.version` to the voice's `ola_mode` flag at trigger time.
+    - **Files**: `src/audio/dct.h`, `src/audio/synth.h`, `src/audio/synth.cc`, `src/audio/window.h`, `src/audio/window.cc`, `src/audio/tracker.cc`
+    - **Tests**: 34/34 passing
+    - **Pending**: Regenerate `.spec` files from MP3 assets to activate v2 encoding.
+
 ## Recently Completed (February 21, 2026)
 
 - [x] **WGSL Refactor: getScreenCoord Helper**