ci(regression): add webm-vp9 to shard-3 + refactor formatExtension

Three follow-ups bundled together (Vai's review feedback on PR #952
plus the fallow audit finding that surfaced when the webm case was
added):

1. **Wire webm-vp9 into CI regression.** The fixture was added in this
   PR but never appeared in any `.github/workflows/regression.yml`
   shard's args allowlist, so the regression harness's positional-args
   gate skipped it in CI. Append `webm-vp9` to shard-3 (which already
   carries `mp4-h264-sdr` + `webm-transparency`) so the fixture runs.

2. **Fix stale "four hard gates" prose in checkDistributedSupport
   docstring.** Earlier in the stack I removed the webm bullet but
   didn't update the count. Two gates remain (fps + hdr).

3. **Refactor `formatExtension` from switch to lookup table.** Adding
   the webm case made the switch dispatch's CRAP score hit 30.0
   (cyclomatic = 5, plus the function's small body). Replaced with a
   `Record<DistributedFormat, string>` lookup, which:
   - drops cyclomatic from 5 → 1,
   - keeps exhaustiveness enforcement at compile time (TS errors if
     a new format gets added to `DistributedFormat` without a
     matching key in the Record literal),
   - drops the runtime `_exhaustive: never` throw, which was only
     guarding against an arbitrary string slipping past TS — a
     caller-side concern, not this function's job.

   The function now reads as a table lookup, which matches what it
   actually does, and the fallow audit now reports zero new
   complexity findings (down from 1).

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

Author: jrusso1020

.github/workflows/regression.yml

@@ -64,7 +64,7 @@ jobs:
           - shard: shard-2
             args: "style-15-prod hdr-hlg-regression style-1-prod many-cuts vfr-screen-recording render-symlinked-assets"
           - shard: shard-3
-            args: "style-7-prod style-8-prod style-10-prod css-spinner-render-compat webm-transparency mp4-h264-sdr"
+            args: "style-7-prod style-8-prod style-10-prod css-spinner-render-compat webm-transparency mp4-h264-sdr webm-vp9"
           - shard: shard-4
             args: "style-16-prod style-9-prod style-17-prod iframe-render-compat variables-prod mp4-h265-sdr"
           - shard: shard-5

packages/producer/src/regression-harness-distributed.ts

@@ -67,7 +67,7 @@ export type DistributedSupportResult = { supported: true } | { supported: false;
 
 /**
  * Decide whether a fixture's `renderConfig` is one the distributed pipeline
- * can actually run. The four hard gates:
+ * can actually run. Two hard gates:
  *
  *   - fps must be `{ num: 24|30|60, den: 1 }`. `DistributedRenderConfig.fps`
  *     accepts only the three integer values, and rationals like