refactor(types): tighten type safety, dedupe HfTransitionMeta, prune dead LUT export (#366)

## Summary

Four small, mechanical type-safety cleanups across `engine`, `producer`, and `shader-transitions`. Zero behavior change — pure pre-cleanup so the rest of the stack ships against a tighter baseline.

## Why

`Chunk 6` of `plans/hdr-followups.md`. Several non-null assertions and a duplicate interface had accumulated as rebase artifacts and leftover work-in-progress; lands first because it touches files later chunks edit and removes friction during review.

## What changed

- `renderOrchestrator.ts`: replace `layers[layerIdx]!` with a `for (const [layerIdx, layer] of layers.entries())` so both index and element come from the iterator.
- `engine/types.ts`: drop the duplicate `HfTransitionMeta` interface (rebase artifact); the original definition above it is the documented one. The orphaned doc comment now precedes `HfProtocol`.
- `shader-transitions/hyper-shader.ts`: keep the local `HfTransitionMeta` declaration (the package ships as a standalone CDN bundle and must not depend on `@hyperframes/engine`), but add a sync comment pointing at the source of truth in `engine/src/types.ts`.
- `alphaBlit.ts` + `engine/index.ts`: drop `export` from `getSrgbToHdrLut` and remove its re-export. It was only ever called by the internal `blitRgba8OverRgb48le`; the public surface was dead code.

## Test plan

- [x] `bun run --filter @hyperframes/engine typecheck`
- [x] `bun run --filter @hyperframes/producer typecheck`
- [x] `bun run --filter @hyperframes/shader-transitions typecheck`
- [x] `bun run --filter @hyperframes/engine test` — 308/308 pass (no test changes; assertions removed in code only).

## Stack

Chunk 6 of `plans/hdr-followups.md`. Mechanical cleanup landed early per the suggested merge order.

packages/shader-transitions/src/hyper-shader.ts

@@ -113,6 +113,9 @@ export function init(config: HyperShaderConfig): GsapTimeline {
     }
   }
 
+  // Locally redeclared (not imported) because @hyperframes/shader-transitions
+  // ships as a standalone CDN bundle and must not depend on @hyperframes/engine.
+  // Keep this in sync with HfTransitionMeta in packages/engine/src/types.ts.
   interface HfTransitionMeta {
     time: number;
     duration: number;