test(shader-transitions): add midpoint (p=0.5) regression invariants for all shaders

[vanceingalls]

Summary

Add four midpoint (p=0.5) regression invariants applied via a describe loop over ALL_SHADERS, so every existing and future shader transition automatically gets coverage at the most viewer-visible point in the animation.

Why

Chunk 9G of plans/hdr-followups.md. Existing smoke tests cover only the endpoints (p=0 ≈ from, p=1 ≈ to), which miss a class of regressions that surface specifically at the midpoint and let shaders silently rot in CI:

• A shader becomes a no-op (returns input as-is)
• A shader prematurely completes (returns target at midpoint)
• A shader doesn't write to the output buffer at all
• A shader loses determinism (Math.random / Date.now / leaked state)

What changed

packages/engine/src/utils/shaderTransitions.test.ts: a single describe loop over ALL_SHADERS that asserts at p=0.5:

1. output ≠ from — catches no-ops
2. output ≠ to — catches premature completion
3. output is non-zero — catches blank output
4. output is deterministic — catches accidental non-determinism

Uses two distinct uniform input colors (40000/30000/20000 vs 10000/10000/10000) so equality checks have distinct byte patterns to compare against. Even shaders that warp UVs (which would be no-ops on uniform input alone) produce mix16(from, to, 0.5) at every pixel, distinct from both inputs.

Test plan

• 60 new tests (4 invariants × 15 shaders), all passing.
• Any new transition added to the registry automatically picks up the same coverage.

Stack

Chunk 9G of plans/hdr-followups.md. Test-only change, independent of all other chunks.

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

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• main

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