feat(producer): add shaderTransitionWorkerPool (hf#732 PR 3/5) (#758)

## Summary

PR 3 of 5 in the hf#732 decomposition stack. Adds a `worker_threads`-based pool that runs the shader-transition blend (one of 15 transition shaders) on a fixed-size worker pool. **No production wiring yet** — the pool stands alone; PR 4 wires it.

The shader blend is a hot inner loop over every pixel of every transition frame at 16bpc. Moving it off the main event loop removes the JS-event-loop ceiling that capped throughput in earlier hf#732 iterations.

### New files

- `packages/producer/src/services/shaderTransitionWorker.ts` — worker entry. Imports from `@hyperframes/engine/shader-transitions` (zero-import TS source).
- `packages/producer/src/services/shaderTransitionWorkerPool.ts` — fixed-size pool. Uses `transferList` so the 16bpc HDR `from`/`to`/`out` buffers move by ownership.
- `packages/producer/src/services/shaderTransitionWorkerPool.test.ts` — 6 vitest tests pinning byte-equivalence across all 15 shaders, transferList correctness, pool lifecycle. All pass.

### Build wiring

- `packages/cli/tsup.config.ts`: third tsup entry emits `dist/shaderTransitionWorker.js`.
- `packages/producer/build.mjs`: fourth esbuild entry for direct producer consumers.
- `packages/engine/package.json`: adds `./shader-transitions` subpath export.

## Stack

Stacked on top of #757 (PR 2: pngDecodeBlit pool). No behavior change in any render.

## Test plan

- [x] 6 pool tests pass
- [x] Producer + engine typecheck clean
- [x] oxlint clean

— Vai

packages/engine/package.json

@@ -12,7 +12,8 @@
   "types": "./src/index.ts",
   "exports": {
     ".": "./src/index.ts",
-    "./alpha-blit": "./src/utils/alphaBlit.ts"
+    "./alpha-blit": "./src/utils/alphaBlit.ts",
+    "./shader-transitions": "./src/utils/shaderTransitions.ts"
   },
   "scripts": {
     "build": "tsc",