feat(engine,cli): drawElement fast-capture config + CLI flag (#1916)

## drawElement fast-capture — config + CLI flag (stack 1/6)

Foundation layer for the drawElement fast-capture feature: the config surface and CLI/Docker plumbing that the rest of the stack builds on.

### What this adds
- **`packages/engine/src/config.ts`** — new config fields for fast capture: `useDrawElement` / `enableDrawElementWorkerEncode` (macOS-GPU `drawElementImage` capture + worker-offloaded JPEG encode), resolved from env in `resolveConfig` (env `HF_DE_WORKER_ENCODE`). Wired alongside main's existing `staticFrameDedup` (unified downstream in 4/6).
- **`packages/cli/src/commands/render.ts`** — `--experimental-fast-capture` flag → sets `experimentalFastCapture`; `--debug` passthrough.
- **`packages/cli/src/utils/dockerRunArgs.ts`** — pass the fast-capture env through to the container.
- **`.github/workflows/fast-video-validation.yml`** — CI job validating fast-capture renders.
- `.oxlintrc.json` / `.fallowrc.jsonc` — ignore-pattern housekeeping for the new paths.

### Notes
- Config-only + entrypoint; no capture behavior yet (that's 2/6–4/6).
- Tests: `config.test.ts`, `dockerRunArgs.test.ts` added.

---
**Stack (drawElement fast-capture, rebased onto current `main`, supersedes #1295 + #1444):**
1. **#1916 config + CLI** ← you are here
2. #1917 drawElementImage capture service
3. #1918 3D projection + compositor-effect risk gate
4. #1919 frame-capture core (routing, worker-encode, static-dedup unification)
5. #1920 producer render stages + remote bg-image localizer
6. #1921 lint rule + player media sync

⚠️ Intermediate PRs (1–5) are split by package boundary for review and **do not each compile independently** (cross-file deps); the complete feature is green at the stack tip (#1921) — tsc-clean on engine + producer, 231 tests pass.

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

.github/workflows/fast-video-validation.yml

@@ -0,0 +1,55 @@
+# Validates the experimental fast-capture (drawElementImage) VIDEO path on a
+# native amd64 Linux runner — where chrome-headless-shell's per-frame BeginFrame
+# drives a real paint each frame, so drawElementImage's snapshot is fresh and
+# video captures correctly. This is the one part of the feature that could not be
+# validated locally (macOS has no BeginFrame; Docker-on-rosetta hung).
+# See docs/fast-capture-limitations.md (Limitation 2).
+#
+# Manual trigger: Actions → "Fast-capture video validation" → Run workflow.
+name: Fast-capture video validation
+
+permissions:
+  contents: read
+
+on:
+  # Manual only. NOTE: currently fails by design — fast capture cannot capture
+  # video on any platform yet (see docs/fast-capture-limitations.md, Limitation 2).
+  # This is the regression gate for if/when fast video is implemented.
+  workflow_dispatch:
+    inputs:
+      composition:
+        description: Test composition to render (must contain <video>)
+        default: sub-composition-video
+      min_psnr:
+        description: Min fast-vs-baseline PSNR (dB) to pass
+        default: "25"
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
+
+      - name: Build test Docker image (cached)
+        uses: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8 # v6
+        with:
+          context: .
+          file: Dockerfile.test
+          load: true
+          tags: hyperframes-producer:test
+          cache-from: type=gha,scope=regression-test-image
+          cache-to: type=gha,mode=max,scope=regression-test-image
+
+      - name: Validate fast-capture video (drawElement + BeginFrame)
+        run: |
+          docker run --rm \
+            --security-opt seccomp=unconfined \
+            --shm-size=4g \
+            -e PRODUCER_VALIDATE_COMP='${{ inputs.composition }}' \
+            -e PRODUCER_VALIDATE_MIN_PSNR='${{ inputs.min_psnr }}' \
+            --workdir /app/packages/producer \
+            --entrypoint bunx \
+            hyperframes-producer:test tsx scripts/validate-fast-video.ts