test(shader-transitions): add midpoint (p=0.5) regression invariants for all shaders (#378)

## 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

- [x] 60 new tests (4 invariants × 15 shaders), all passing.
- [x] 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.

packages/engine/src/services/chunkEncoder.test.ts

@@ -460,6 +460,7 @@ describe("buildEncoderArgs HDR color space", () => {
       expect.stringContaining("HDR is not supported with codec=h264"),
     );
     warnSpy.mockRestore();
+
   });
 
   it("uses range conversion for HDR CPU encoding", () => {