PHLEGM v6 - SNES PPU prototype compiler fix - 2026-10-04

Compiler run 2026-1004-1130-3429 failed all four Windows targets in the shared SNES PPU read path.

Root cause:
- wide-support-devices.bi defines SnesPpuCpuRead(registerOffset, externalBus), and native-support.bi correctly calls it with gCpuDataBusLatch.
- PHLEGM.bas still forward-declared the older one-argument SnesPpuCpuRead(registerOffset) signature.
- FreeBASIC therefore rejected the two-argument definition/call and emitted follow-on externalBus parser errors.

Fix:
- Update the PHLEGM.bas forward declaration to include ByVal externalBus As UByte.
- Preserve the external CPU data-bus latch behavior for write-only/open-bus SNES PPU reads.
- Extend validate_snes_support_undocumented.py to require the matching two-argument forward declaration.

Validation:
- All 23 Python validators pass after the correction.
- The matching SNES support validator explicitly checks the declaration, definition behavior, and gCpuDataBusLatch call path.
- Native FreeBASIC compilation still requires the Windows FBC environment.
