feat(audio): OLA-IDCT synthesis with Hann window to eliminate clicks

Add v2 spectrogram format (SPEC_VERSION_V2_OLA) using overlap-add IDCT
with 50% overlap (hop=256, OLA_OVERLAP=256) and Hann windowing.

- dct.h: OLA_HOP_SIZE=256, OLA_OVERLAP=256
- synth.h: SPEC_VERSION_V1/V2_OLA constants; version field on Spectrogram
- window.h/cc: hann_window_512() alongside existing hamming_window_512()
- synth.cc: g_hann[] precomputed at init; OLA path in synth_render when
  ola_mode=true (IDCT -> Hann -> add overlap tail -> save new tail ->
  output OLA_HOP_SIZE samples); v1 path unchanged for backward compat
- tracker.cc: MP3 encoder now uses sliding 512-sample Hann window with
  OLA_HOP_SIZE advance per frame; sets version=SPEC_VERSION_V2_OLA;
  .spec files propagate header->version; generated notes stay v1

Existing .spec files must be regenerated to benefit from click-free OLA.

handoff(Claude): OLA done. .spec files need regen via MP3 tool to activate v2.

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