PHLEGM v6 - NES PPU Skip Diagnostic Toggle - 2026-10-03

Adds a runtime NES PPU rendering diagnostic beside the existing APU diagnostic.

GUI layout:
  PPU Skip: Off/On   |   APU Skip: Off/On

PPU Skip is intentionally left of APU Skip. Both default Off.

PPU Skip leaves RP2C02 timing active: scanline/dot timing, VBlank, NMI, $2002,
and sprite-zero hit scheduling continue. Full background/sprite rasterization and
presentation-buffer rebuilding are skipped. A lightweight sprite-0 overlap check
preserves CPU-visible sprite-zero timing without drawing the complete frame.
The last completed frame remains visible while frameNumber continues so the FPS
overlay can still be used for the performance comparison.
