fix(audio): correct OLA synthesis and extract shared ola_encode/ola_decode

- Remove erroneous Hann synthesis window from synth.cc (g_hann * tmp[j]).
  Hann analysis at 50% overlap satisfies w[n]+w[n+H]=1, so rectangular
  synthesis gives perfect reconstruction; applying Hann twice was wrong.
- Extract ola_encode()/ola_decode()/ola_num_frames() into src/audio/ola.h+cc.
  spectool and test_wav_roundtrip now use the shared functions.
  synth.cc lazy-decode path stays inlined (see TODO for future refactor).
- Drop dead <atomic> include and g_hann array from synth.cc.
- Drop dead window.h include from spectool.cc.
- Update PROJECT_CONTEXT.md, COMPLETED.md, TODO.md to reflect correct
  analysis-only Hann window and new ola.h API.

handoff(Gemini): OLA synthesis bug fixed + ola.h factorized. synth.cc
lazy-decode still inline (TODO item added). 34/35 tests pass; WavDumpBackendTest
failure is pre-existing and unrelated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/PROJECT_CONTEXT.md b/PROJECT_CONTEXT.md
index d9e37e8..690b589 100644
--- a/PROJECT_CONTEXT.md
+++ b/PROJECT_CONTEXT.md
@@ -12,7 +12,7 @@
 ## Audio
 - 32 kHz, 16-bit stereo
 - Procedurally generated samples
-- Real-time additive synthesis from spectrograms (OLA-IDCT, Hann window, 50% overlap)
+- Real-time additive synthesis from spectrograms (OLA-IDCT, Hann analysis window, 50% overlap, rectangular synthesis)
 - Variable tempo system with music time abstraction
 - Event-based pattern triggering for dynamic tempo scaling
 - Modifiable Loops and Patterns, w/ script to generate them (like a Tracker)
@@ -33,7 +33,7 @@
 
 - **Timing System:** **Beat-based timelines** for musical synchronization. Sequences defined in beats, converted to seconds at runtime. Effects receive both physical time (constant) and beat time (musical). Variable tempo affects audio only. See `doc/BEAT_TIMING.md`.
 - **Workspace system:** Multi-workspace support. Easy switching with `-DDEMO_WORKSPACE=<name>`. Organized structure: `music/`, `weights/`, `obj/`, `shaders/`. Shared common shaders in `src/shaders/`. See `doc/WORKSPACE_SYSTEM.md`.
-- **Audio:** Sample-accurate sync. Zero heap allocations per frame. Variable tempo. OLA-IDCT synthesis (v2 .spec): Hann window, 50% overlap, click-free. V1 (raw DCT-512) preserved for generated notes. .spec files regenerated as v2.
+- **Audio:** Sample-accurate sync. Zero heap allocations per frame. Variable tempo. OLA-IDCT synthesis (v2 .spec): Hann analysis window, rectangular synthesis, 50% overlap, click-free. V1 (raw DCT-512) preserved for generated notes. .spec files regenerated as v2.
 - **Shaders:** Parameterized effects (UniformHelper, .seq syntax). Beat-synchronized animation support (`beat_time`, `beat_phase`). Modular WGSL composition with ShaderComposer. 24 shared common shaders (math, render, compute).
 - **3D:** Hybrid SDF/rasterization with BVH. Binary scene loader. Blender pipeline.
 - **Effects:** CNN post-processing: CNNEffect (v1) and CNNv2Effect operational. CNN v2: sigmoid activation, storage buffer weights (~3.2 KB), 7D static features, dynamic layers. Training stable, convergence validated.