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

• docs(engine): document __name polyfill and add regression test #385
• perf(producer): cache transfer-converted hdr image buffers per render job #384
• perf(producer): gate per-frame debug meta via optional isLevelEnabled #383
• perf(producer): hdr benchmark harness — --tags filter, peak heap/RSS tracking, bench:hdr script #382
• test(producer): extract frameDirMaxIndexCache to its own module and pin cross-job isolation #381
• test(engine): cover spawnStreamingEncoder lifecycle and cleanup paths #380
• test(engine): add ffprobe-unavailable fallback regression tests #379
• test(shader-transitions): add midpoint (p=0.5) regression invariants for all shaders #378 👈 (View in Graphite)
• test(engine): lock down sRGB→BT.2020 LUT with byte-exact reference values #377
• build(lfs): track tests/*/src/*.png via Git LFS #376
• test(hdr-regression): tighten Window F maxFrameFailures budget after Chunk 4 fix #375
• fix(engine,shader): handle matrix3d transforms and hide non-first scenes #374
• refactor(producer): extract HDR compositing helpers and rename media metadata #373
• fix(producer): wire --crf and --video-bitrate CLI overrides into encoders #372
• fix(producer): tighten resource lifecycle and harden file server #371
• feat(engine): wire options.hdr through chunkEncoder + dynamic SDR→HDR transfer #370
• test(hdr-regression): tighten Window C maxFrameFailures budget after Chunk 1 fix #369
• fix(engine): stop clobbering native <video> opacity in HDR pipeline #368
• refactor(shader-transitions): extract DEFAULT_DURATION and DEFAULT_EASE constants #367
• refactor(types): tighten type safety, dedupe HfTransitionMeta, prune dead LUT export #366
• test(producer): add hdr-regression and hdr-hlg-regression test suites #365
• main

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[jrusso1020]

Midpoint (p=0.5) regression invariants are the right surface to pin for a shader-transitions library. Shaders at p=0 and p=1 have strong invariants (equal to fromScene / toScene respectively), but the middle is where actual transition logic runs — wipe direction, crossfade alpha, shader-specific parameters. Without a p=0.5 pin, a shader rewrite can flip direction, swap source/destination, or change easing curve without being detected by the existing endpoint tests.

"Invariants for all shaders" is the right scope — a single shader-by-shader snapshot would be noisy and brittle, but invariant assertions (e.g. "the midpoint output is a mix of both sources" or "no pixel is fully black") are durable and meaningful.

Approved.

— Rames Jusso

[vanceingalls]

Thanks for the approval @jrusso1020.

You nailed the rationale — endpoint pins (p=0 ≡ fromScene, p=1 ≡ toScene) are strong but say nothing about the actual transition logic. The midpoint is where direction, crossfade alpha, and shader-specific parameters live, so that's the surface a shader rewrite is most likely to silently break.

The "invariants over snapshots" framing is what kept the harness durable: assertions like "midpoint output is a non-trivial mix of both sources", "no pixel is fully black/saturated", and "midpoint is symmetric for crossfade-class shaders" survive shader refactors that don't change semantics, but reliably trip on direction flips, source/destination swaps, or easing-curve regressions.

No outstanding action items from the review.