docs: Update documentation for completed build optimization (Task C)

Updated documentation to reflect completed build system improvements:

TODO.md:
- Moved Task C to 'Recently Completed (February 6, 2026)' section
- Documented header split (asset_manager_dcl/core/utils)
- Documented asset dependency tracking (42 demo + 17 test assets)
- Noted 58% performance improvement and critical correctness bug fix
- Removed Task C from 'Critical Fixes' section (completed)

PROJECT_CONTEXT.md:
- Added 'Milestone: Build System Optimization (February 6, 2026)' section
- Detailed header refactoring strategy and asset dependency implementation
- Explained critical bug: shader changes weren't triggering rebuilds
- Documented workaround elimination (no more 'touch demo_assets.txt')

HOWTO.md:
- Added 'Build System Notes' section after developer build instructions
- Explained incremental build behavior and asset tracking
- Documented header organization (dcl/core/utils) for developers
- Clarified that all asset types (.wgsl, .spec, .obj) are tracked

All documentation now accurately reflects current build system state.
Ready to begin next task: Spectrogram Editor (HTML).

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