feat(sequence): complete phase 3 - v2 shader integration and effect ports

- Create v2-compatible WGSL shaders with UniformsSequenceParams
- Add sequence_v2_uniforms snippet for ShaderComposer
- Port 3 effects: PassthroughEffectV2, GaussianBlurEffectV2, HeptagonEffectV2
- Enable and fix end-to-end test (test_sequence_v2_e2e)
- Fix shader binding order (sampler at 0, texture at 1)
- Fix WebGPU validation (maxAnisotropy=1, explicit depthSlice)
- Add v2 shaders to main and test workspace assets
- All tests passing (36/36)

handoff(Claude): Phase 3 complete, v2 effects functional, ready for phase 4

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