SPECTRE/SHACHI_PROBE_PLAN.md (159 lines)

• SHACHI-PROBE: PVR2 divergence test ROM — single KOS program with menu system
• Tests 7 of 10 documented Flycast/hardware divergences (DIV-001 to DIV-008):
  • DIV-001: Near-Z clipping (CRITICAL — hardware renders garbage at Z→0)
  • DIV-002: Depth precision (Z-fighting at far range from 1/W precision loss)
  • DIV-003: Twiddle bit order (Morton order: Y-even, X-odd)
  • DIV-004: Transparency sort order (OIT in Flycast vs hardware)
  • DIV-005: OPB overflow (hardware drops geometry)
  • DIV-007: Performance masking (SH-4 falls off 60fps threshold)
  • DIV-008: Mixed SQ/DMA submission (hardware shows corruption)
• Reusable debug overlay module shachi_debug.h/c reads 10 PVR2 registers via PVR_GET()
• Project structure: ~1060 lines total across 14 files
• Menu system: numbered list + debug overlay toggle
• Shared utilities: probe_draw_quad, probe_draw_textured_quad, probe_draw_text, probe_twiddle, probe_twiddle_swapped