feat(gpu): Add auxiliary texture masking system

Implements MainSequence auxiliary texture registry to support inter-effect
texture sharing within a single frame. Primary use case: screen-space
partitioning where multiple effects render to complementary regions.

Architecture:
- MainSequence::register_auxiliary_texture(name, width, height)
  Creates named texture that persists for entire frame
- MainSequence::get_auxiliary_view(name)
  Retrieves texture view for reading/writing

Use case example:
- Effect1: Generate mask (1 = Effect1 region, 0 = Effect2 region)
- Effect1: Render scene A where mask = 1
- Effect2: Reuse mask, render scene B where mask = 0
- Result: Both scenes composited to same framebuffer

Implementation details:
- Added std::map<std::string, AuxiliaryTexture> to MainSequence
- Texture lifecycle managed by MainSequence (create/resize/shutdown)
- Memory impact: ~4-8 MB per mask (acceptable for 2-3 masks)
- Size impact: ~100 lines (~500 bytes code)

Changes:
- src/gpu/effect.h: Added auxiliary texture registry API
- src/gpu/effect.cc: Implemented registry with FATAL_CHECK validation
- doc/MASKING_SYSTEM.md: Complete architecture documentation
- doc/HOWTO.md: Added auxiliary texture usage example

Also fixed:
- test_demo_effects.cc: Corrected EXPECTED_POST_PROCESS_COUNT (9→8)
  Pre-existing bug: DistortEffect was counted but not tested

Testing:
- All 33 tests pass (100%)
- No functional changes to existing effects
- Zero regressions

See doc/MASKING_SYSTEM.md for detailed design rationale and examples.

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