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, 2 insertions, 1 deletions

diff --git a/src/tests/gpu/test_spectool.cc b/src/tests/gpu/test_spectool.cc
index 984322a..b90d236 100644
--- a/src/tests/gpu/test_spectool.cc
+++ b/src/tests/gpu/test_spectool.cc
@@ -3,6 +3,7 @@
 // Generates a test WAV, analyzes it, and verifies the resulting .spec file.
 
 #include "audio/audio.h"
+#include "audio/synth.h"
 #include <assert.h>
 #include <math.h>
 #include <stdio.h>
@@ -54,7 +55,7 @@ int main() {
   size_t read = fread(&header, sizeof(SpecHeader), 1, f);
   assert(read == 1);
   assert(strncmp(header.magic, "SPEC", 4) == 0);
-  assert(header.version == 1);
+  assert(header.version == SPEC_VERSION_V2_OLA);
   assert(header.dct_size == 512);
   assert(header.num_frames > 0);