PHLEGM v6 NES APU Bypass Diagnostic Toggle - 2026-10-03

Adds an always-visible runtime diagnostic button to the Control / Monitor GUI.

Modes:
  APU Skip: Off  - default; normal NES APU advancement, DMC stalls, and native PCM.
  APU Skip: On  - diagnostic only; freezes NES APU advancement, bypasses DMC-generated
                 CPU stalls and native PCM sample-boundary work while CPU, PPU,
                 controller, and APU register I/O remain active.

The toggle does not change .phlegm data and does not alter Commodore or SNES support
chips. Switching modes clears queued native PCM and rebases the host pacing anchor so
the button click itself cannot contaminate the FPS comparison. The GUI status pane
shows NES APU diagnostic: BYPASS while the diagnostic mode is active.

This mode is intended only to isolate APU cost. It is not a valid emulation mode: while
bypassed, APU/DMC state intentionally stops advancing.
