feat(spectool): add --wav decode, IMDCT, and roundtrip test

- spectool --wav <input.spec> <output.wav>: decodes .spec to mono 16-bit
  WAV at 32 kHz using IDCT-OLA synthesis (no synthesis window).
  The analysis Hann window at 50% overlap satisfies w[n]+w[n+H]=1,
  so the synthesis window must be rectangular for perfect reconstruction.

- Add imdct_512 / imdct_fft to audio lib (fft.cc, fft.h, idct.cc, dct.h)
  for future MDCT-based synthesis.

- test_wav_roundtrip: in-process OLA analyze+decode SNR test (≥30 dB).
  Currently measures 53 dB on a 440 Hz sine.

- Fix stale test_spectool.cc: version assertion updated from 1 to
  SPEC_VERSION_V2_OLA (was always wrong since OLA fix landed).

- Docs: TOOLS_REFERENCE.md removes dead specview, documents --wav /
  --normalize / test_gen. HOWTO.md adds decode section. TRACKER.md
  notes spec v2 OLA format and decode command.

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

1 files changed, 11 insertions, 0 deletions

diff --git a/doc/HOWTO.md b/doc/HOWTO.md
index 768e51d..cc43e7c 100644
--- a/doc/HOWTO.md
+++ b/doc/HOWTO.md
@@ -257,6 +257,17 @@ cmake --build build -j4 --target spectool
 
 Requires `ffmpeg` for `.aif` input files. Output uses v2 format (OLA, Hann, hop=256).
 
+### Decoding .spec to WAV
+
+To verify or inspect a `.spec` file as audio:
+
+```bash
+./build/spectool --wav input.spec output.wav
+```
+
+Produces a mono 16-bit PCM WAV at 32 kHz using IDCT-OLA synthesis (no synthesis window —
+the analysis Hann window is its own reconstruction filter at 50% overlap).
+
 ---
 
 ## Assets