test(producer): add variables-prod regression test for the variables stack

End-to-end Docker regression test that exercises the full variables
chain: meta.json renderConfig.variables → harness → createRenderJob →
RenderConfig → CaptureOptions → engine evaluateOnNewDocument →
window.__hfVariables → getVariables() → DOM text → rendered pixels.

Fixture (packages/producer/tests/variables-prod/):
- src/index.html: composition with three declared variables (title,
  subtitle, bgColor) read via window.__hyperframes.getVariables() and
  rendered as positioned text on a colored background. No animation —
  keeps the regression frame-stable so it isolates "did the variables
  flow through?" from motion concerns.
- meta.json: tags ["variables", "composition"] (runs in the existing
  fast shard's tag filter), renderConfig.variables provides override
  values the baseline reflects ("Override Title", "Override subtitle",
  #0a3d62). Defaults would produce a visibly different frame, so a
  failing baseline that reflects defaults means the variables didn't
  propagate.
- output/output.mp4: Docker-generated baseline per the project's
  CLAUDE.md golden-baseline rule.

Harness change (packages/producer/src/regression-harness.ts):
- TestMetadata.renderConfig gains an optional variables field,
  validated as a JSON object in the meta.json validator.
- The createRenderJob call site forwards renderConfig.variables to
  RenderConfig.variables, which the engine already consumes via
  evaluateOnNewDocument (PR #600).

Verified: docker:test variables-prod passes 100/100 visual checkpoints
and audio correlation 1.000 against the committed baseline.

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

packages/producer/tests/variables-prod/meta.json

@@ -0,0 +1,17 @@
+{
+  "name": "Variables Prod",
+  "description": "End-to-end regression for the variables system. Composition declares title/subtitle/bgColor in data-composition-variables and reads them via window.__hyperframes.getVariables(). renderConfig.variables provides overrides that flow CLI → producer → engine evaluateOnNewDocument → window.__hfVariables → helper merge → DOM text → rendered pixels. If any link in that chain breaks, the rendered text/background diverges from the baseline and PSNR fails.",
+  "tags": ["variables", "composition"],
+  "minPsnr": 30,
+  "maxFrameFailures": 0,
+  "minAudioCorrelation": 0.9,
+  "maxAudioLagWindows": 120,
+  "renderConfig": {
+    "fps": 24,
+    "variables": {
+      "title": "Override Title",
+      "subtitle": "Override subtitle",
+      "bgColor": "#0a3d62"
+    }
+  }
+}