Godot host: source images kept per slot with dims read from the BMP header on
the VM thread (synchronous, so the bytecode's geometry math sees real sizes);
draw-texture blits src rect -> dst into one 800x600 canvas in execution order,
shown by a single TextureRect. Selftest byte-parity preserved.
Slot 0 = primary/screen surface: seed its dims to 800x600 (the boot-time
create-texture the single-scene harness skips) and record create-texture(w,h),
so the first CG's anchor-preserve math stays an identity instead of corrupting
the persistent base globals. Fixes the grey-first-CG opening. Verified: SC0000
pages 1/3 composite the opening event CG at (0,0) with dialogue over it.
Also adds a --shot <png> [--shot-page N] dev capture to Main (page-gated on a
VM-thread counter) for headless visual verification.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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