PHLEGM v6 NES APU Fidelity Fix - 2026-10-03

Scope
-----
Host reconstruction of the existing Ricoh RP2A03 APU output. CPU/APU timer periods,
frame sequencing, DMC timing, machine pacing, and the 44.1-kHz WinMM output rate are
not retuned by this checkpoint.

Changes
-------
1. Native NES DAC observation is now 4x the host PCM rate (176.4 kHz at the canonical
   44.1-kHz output). Four machine-time observations are averaged into each output
   sample. This suppresses the strong alias products created by direct point sampling
   of pulse/noise edges while keeping host PCM at exactly 44.1 kHz.

2. The nonlinear pulse and triangle/noise/DMC mixer equations are cached as exact
   lookup tables. The oversampled hot path therefore does not add floating-point
   division at every observation.

3. Native NES PCM receives the console's nominal analog reconstruction chain before
   the user-selected final transducer model: first-order high-pass stages near 90 Hz
   and 440 Hz followed by a first-order low-pass stage near 14 kHz.

4. The prior v6 native-runtime cache, integer observation-boundary countdown, and
   asynchronous NES presentation fixes remain intact. PPU Skip and APU Skip remain
   diagnostic-only and default Off.

Why
---
After the native hot-path repair restored Donkey Kong to 59-61 FPS, remaining audio
was still fuzzy and subjectively off-pitch. The timer formulas already match the NTSC
2A03 relationships; the remaining host path was observing discontinuous NES DAC state
directly at 44.1 kHz with no anti-alias reconstruction. The new 4x observation/average
path targets that host-audio artifact without changing emulated machine time.

Validation
----------
- All checkpoint validate_*.py checks pass, including validate_nes_audio_fidelity.py.
- Exact nonlinear mixer equations are preserved in lookup-table initialization.
- WinMM remains mono 16-bit PCM at NATIVE_AUDIO_SAMPLE_RATE (44100 Hz).
- fbc/Wine are not installed in the validation container, so Windows runtime testing
  remains the end-user verification step.
