docs: Add auxiliary texture initialization tech doc

Documents the "half resolution" bug, root cause analysis, and
solution decision (resize before init vs lazy initialization).

Key points:
- Problem: Auxiliary textures created with default dimensions
- Root cause: init() called before resize()
- Solution: Swap order (resize → init) for 2-line fix
- Rejected: Lazy initialization (too complex, cascade effects)

Includes implementation details and guidelines for new effects.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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