refactor(producer): extract HDR compositing helpers and rename media metadata (#373)

## Summary

Four behavior-preserving refactors that reduce complexity in `renderOrchestrator.ts` and clarify the engine ffprobe utility surface. Lands after the correctness fixes (Chunks 1–5) so the refactored code is already correct.

## Why

`Chunk 7` of `plans/hdr-followups.md`. The HDR composite block had grown a ~200 LOC inline closure with 14 captured deps, a repeated capture-options spread, a `extractVideoMetadata` name that now also handles still images, and per-frame re-creation of debug helpers.

## What changed

**7A — Hoist `compositeToBuffer` into a module-scoped helper.** Extract the inline HDR closure into a top-level `compositeHdrFrame()` that takes an `HdrCompositeContext` struct. Construct the context once at the top of the HDR render block and pass it through. Removes a deeply-nested closure from the middle of the orchestrator.

**7B — `buildHdrCaptureOptions()` helper.** Factor the repeated `{ ...captureOptions, skipReadinessVideoIds: ... }` spread into a named helper at the call site.

**7C — Rename `extractVideoMetadata` → `extractMediaMetadata`.** Reflects that the helper handles still images (PNG/JPEG/WebP) in addition to video. Update all callers in engine + producer (`videoFrameExtractor`, `htmlCompiler`, regression-harness, producer ffprobe re-export, tests). Re-export the old name as a deprecated alias from `@hyperframes/engine` for backward compatibility, plus the producer re-export shim.

**7D — Hoist debug counters to module scope.** `countNonZeroAlpha` and `countNonZeroRgb48` are now module-scoped so they aren't re-created per frame and so the closure has fewer captures.

Also touches the `hdr-regression` and `hdr-hlg-regression` README + `meta.json` files reviewed during this refactor.

## Test plan

- [x] `bunx tsc --noEmit -p packages/producer && bunx tsc --noEmit -p packages/engine` clean.
- [x] Engine tests: 313 pass, 0 fail (1218 expect calls).
- [x] `bunx oxlint` + `bunx oxfmt --check` clean on 8 changed source files.
- [x] Diff is structural only — no behavioral changes.

## Stack

Chunk 7 of `plans/hdr-followups.md`. Lands after the correctness fixes (Chunks 1–5) per the suggested merge order.

packages/engine/src/utils/ffprobe.test.ts

@@ -1,7 +1,7 @@
 import { readFileSync } from "fs";
 import { resolve } from "path";
 import { describe, expect, it } from "vitest";
-import { extractPngMetadataFromBuffer, extractVideoMetadata } from "./ffprobe.js";
+import { extractMediaMetadata, extractPngMetadataFromBuffer } from "./ffprobe.js";
 
 function crc32(buf: Buffer): number {
   let crc = 0xffffffff;
@@ -51,14 +51,14 @@ function buildMinimalPng(options?: {
     : buildPngWithChunks([ihdr, cicp, idat, iend]);
 }
 
-describe("extractVideoMetadata", () => {
+describe("extractMediaMetadata", () => {
   it("reads HDR PNG cICP metadata when ffprobe color fields are absent", async () => {
     const fixturePath = resolve(
       __dirname,
       "../../../producer/tests/hdr-regression/src/hdr-photo-pq.png",
     );
 
-    const metadata = await extractVideoMetadata(fixturePath);
+    const metadata = await extractMediaMetadata(fixturePath);
 
     expect(metadata.colorSpace).toEqual({
       colorPrimaries: "bt2020",