ci(regression): add webm-vp9 to shard-3 + fix harness doc count

Three follow-ups bundled together (Vai's review feedback on PR #952
plus the fallow audit findings that surfaced when committing them):

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. **Suppress fallow complexity finding on `formatExtension`.**
   Adding the webm case tipped the function's CRAP score from 4
   (passing) to 5 (= threshold 30.0), tripping the pre-commit audit.
   The function is an exhaustive switch dispatch over a closed-set
   enum: each case is a literal `return`, and the `default` arm's
   `_exhaustive: never` assertion is load-bearing for future format
   additions. Marked with `fallow-ignore-next-line complexity` per
   the tool's own suggested suppression placement. Investigation log:
   - Fallow's "32 above threshold" output bundles all findings in
     changed files (including 31 inherited). Verdict math is correct.
   - JSON output confirmed `complexity_introduced: 1` →
     `formatExtension` was the lone new finding.
   - With the suppression in place: `complexity_introduced: 0`,
     verdict `warn`, exit 0 on `--fail-on-issues`.

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/aws-lambda/src/formatExtension.ts

@@ -8,6 +8,12 @@
 
 export type DistributedFormat = "mp4" | "mov" | "png-sequence" | "webm";
 
+// Exhaustive switch dispatch over `DistributedFormat`'s closed set.
+// Adding the webm branch tipped the CRAP score to the 30.0 threshold;
+// each case is a literal return and the `default` arm's
+// `_exhaustive: never` assertion is load-bearing for future format
+// additions.
+// fallow-ignore-next-line complexity
 export function formatExtension(format: DistributedFormat): string {
   switch (format) {
     case "mp4":

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