fix(audio): Scale procedural note generation for orthonormal DCT

Fixed procedural notes (NOTE_*) being inaudible by adding scaling
compensation in gen.cc.

Root Cause:
- Old non-orthonormal DCT produced values ~16x larger (no sqrt scaling)
- New orthonormal DCT: output *= sqrt(1/N) or sqrt(2/N)
- Procedural note generation in gen.cc now produces 16x smaller spectrograms
- IDCT expects same magnitude as .spec files -> notes too quiet

Solution:
- Added scale_factor = sqrt(DCT_SIZE / 2) = sqrt(256) = 16
- Multiply DCT output by 16 to match old magnitude
- Procedural notes now have same loudness as sample-based notes

Verification:
- Checked spectral_editor: does not use DCT for procedural
- Checked editor tools: no procedural generation with DCT
- All 23 tests pass

Procedural notes should now be audible at correct volume.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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