refactor(producer): /simplify Phase 4 distributed-rendering changes

Address findings from a three-agent code-review pass over the Phase 4 stack:

- regression-harness: hoist `readdirSync` out of the per-checkpoint
  failure-extraction loop (was running 20 redundant syscalls on every
  failing png-sequence test). Drop redundant `existsSync` guards before
  `mkdirSync(recursive: true)` and `rmSync(force: true)`. Replace the
  three-deep ternary that built the output filename suffix with a
  single `Record<format, ext>` lookup.
- regression-harness-distributed: flatten the `format === "mp4" ? {...} : {...}`
  branching in the `plan()` call into a single config object with a
  conditional spread. `plan()` already accepts `codec: undefined` for
  non-mp4 formats, so the duplicate object was unnecessary.
- chunkBoundary.test: rename the stale "byte-identical mp4" test title
  to "byte-identical frames" (the test now uses png-sequence). Trim the
  10-line comment justifying `rejectOnSystemFonts: false` to the
  essential WHY.
- renderChunk / plan.test / regression-harness: drop trailing-edge
  comment phrases that pinned the prose to the PR's calendar context
  ("today", "v1.5", "pre-codec-knob output", section-numbered cross-
  references to the planning doc).

No behavior change. All 49 distributed unit tests pass. Smoke + four
distributed format fixtures pass in --mode=distributed-simulated.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

packages/producer/src/services/distributed/plan.test.ts

@@ -242,9 +242,8 @@ describe("plan() — codec knob", () => {
         readFileSync(join(planDir, "meta", "encoder.json"), "utf-8"),
       ) as Record<string, unknown>;
       expect(encoder.encoder).toBe("libx265-software");
-      // SDR 8-bit yuv420p, same as h264. Distributed mode is SDR-only —
-      // anyone reading this and tempted to bump to 10-bit, that's HDR
-      // territory and lives in v1.5.
+      // SDR 8-bit yuv420p, same as h264 — distributed mode is SDR-only and
+      // 10-bit / HDR pixelFormat selection is not exposed on this surface.
       expect(encoder.pixelFormat).toBe("yuv420p");
     },
     TIMEOUT_MS,