fix(producer): address PR review feedback on harness mode + plan() copy filter

Miguel (approved) and Vai (commented) both flagged the same
PSNR-threshold doc/code mismatch; Vai additionally flagged a
path-anchoring bug in the projectDir-copy filter and a dishonest type
cast. Addressed all five findings:

PSNR threshold doc/code mismatch (important):
- Module docstring, `resolveMinPsnrForMode` JSDoc, and tests/README.md all
  claimed distributed-simulated tightens to ≥50 dB. The actual code uses
  `max(fixture.minPsnr, 10)` — 10 dB is a pathology floor, the per-test
  gate is the fixture's authored `minPsnr`. Updated all three doc sites
  to describe what the code does. The 50 dB target in §5.1 is a per-
  render distributed-vs-in-process contract; against the frozen baseline
  it's unreachable for either mode (shared encoder/JPEG jitter), so it
  can't be a per-fixture gate.

`PLAN_PROJECT_DIR_COPY_SKIP` regex matched absolute paths (important):
- `cpSync` calls the filter with the absolute source path, so a
  `projectDir` whose absolute path happens to contain a blocklisted
  segment (`/home/user/work/output/comp/`, `~/projects/dist/foo/`, etc.)
  caused the filter to return false for every descendant — empty
  compiled directory, broken render. Now matches relative-to-projectDir
  segments via `path.relative()` + `split(sep)`. Switched from a regex
  to a Set for clarity. Harness fixtures don't hit this because they
  live under `tests/<name>/src/`, but adapters call `plan()` with
  caller-supplied paths.

Dishonest type cast in regression-harness.ts (important):
- `as "mp4" | "mov" | "png-sequence"` claimed reachability for formats
  that `validateMetadata` doesn't accept (the schema is `"mp4" | "webm"`,
  and webm is rejected by `checkDistributedSupport`). Narrowed to
  hardcoded `format: "mp4"` with a comment naming the metadata-schema
  invariant that lets us do that.

Renamed `chunkVideoInjectorFactory` (nit):
- The variable was invoked once and never used again — "factory" implied
  repeated calls. Inlined as a plain `videoInjector: BeforeCaptureHook | null`
  ternary.

Replaced tautology test (nit):
- `expect(DISTRIBUTED_SIMULATED_MIN_PSNR_DB).toBe(10)` was a value-pin
  over an exported constant. The invariant the JSDoc actually asserts is
  "10 dB is below any real fixture's authored minPsnr"; if someone lands
  a permissive fixture (minPsnr: 5), the value-pin doesn't catch it.
  Replaced with a test that walks `tests/*/meta.json` and asserts every
  authored `minPsnr` is ≥ the floor.

Validated in `docker:test --mode=distributed-simulated`:
  font-variant-numeric, many-cuts, gsap-letters-render-compat,
  style-1-prod, sub-composition-video — all PASSED.
Unit tests: 15/15 pass (new fixture-scan test included).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/producer/tests/README.md

@@ -96,11 +96,16 @@ passing in the summary):
 - `format === "webm"` — `plan()` refuses webm.
 - `hdr === true` — distributed mode is SDR-only at v1.
 
-For fixtures that *are* supported, the PSNR threshold tightens to **≥ 50
-dB** (the §5.1 determinism contract) or the fixture's own `minPsnr`,
-whichever is higher. A failure at this threshold means the distributed
-pipeline has drifted from in-process output — file an issue rather than
-adjusting the threshold.
+Both modes use the fixture's authored `minPsnr` as the per-test
+threshold — distributed must clear the same quality bar in-process
+clears against the same frozen baseline. (`DISTRIBUTED-RENDERING-PLAN.md`
+§5.1's 50 dB target is a per-render distributed-vs-in-process contract;
+against the frozen baseline file, neither mode reaches it consistently
+due to shared encoder/JPEG-capture jitter.) An absolute 10 dB pathology
+floor catches fully-black-output regressions when a fixture authors a
+permissive threshold. A distributed failure at the fixture's own
+threshold means the distributed pipeline has drifted — file an issue
+rather than relaxing the fixture.
 
 `--update` is incompatible with `--mode=distributed-simulated`: the
 in-process renderer is the source of truth for baselines, and the
@@ -124,10 +129,10 @@ bun run --cwd packages/producer docker:test font-variant-numeric -- --mode=distr
 bun run --cwd packages/producer docker:test many-cuts -- --mode=distributed-simulated
 ```
 
-Both modes must produce PSNR ≥ 50 dB against the existing baseline. If
-`--mode=distributed-simulated` fails on a baseline, the distributed
-primitive has a regression — stop, file an issue, do not paper over it
-by adjusting the threshold.
+Both modes must pass at each fixture's authored `minPsnr` against the
+existing baseline. If `--mode=distributed-simulated` fails where
+`--mode=in-process` passes, the distributed primitive has a regression —
+file an issue rather than relaxing the fixture's threshold.
 
 ## Distributed-only fixtures