docs: consolidate and sync docs with current codebase state

- PROJECT_CONTEXT.md: fix effect count (12→18), shader count (27→37),
  update CNN v3 pipeline description, tighten Next Up section
- TODO.md: fix priority numbering, restore GPU PCM synthesis as pending,
  streamline CNN v3 section, consolidate Future items
- doc/SEQUENCE.md: effect count 12→18
- cnn_v3/README.md: phases 1–7→1–9, test count 36→38, add phases 8–9
- cnn_v3/docs/HOWTO.md: fix dataset layout blender/photos→full/simple,
  update test counts 36→38 throughout
- doc/COMPLETED.md: archive FFT/timing/OLA fixes, remove false GPU PCM claim
- src/audio/audio_engine.cc: fix step comment numbering (6→5)
- src/audio/synth.cc: remove stale fractional_pos tempo-scaling comment

handoff(Gemini): docs now accurate — 18 effects, 37 shaders, 38/38 tests,
GPU PCM synthesis back in TODO as pending, CNN v3 dataset layout corrected.

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