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)

packages/cli/src/commands/render.ts

@@ -358,6 +358,18 @@ export default defineCommand({
         "memory thrash on constrained machines. Default: auto-detected from " +
         "total RAM (<= 8 GB). Env: PRODUCER_LOW_MEMORY_MODE.",
     },
+    "experimental-fast-capture": {
+      type: "boolean",
+      description:
+        "EXPERIMENTAL. Capture frames via Chrome's drawElementImage API " +
+        "instead of Page.captureScreenshot — reads DOM paint records directly, " +
+        "~46% faster on GPU. Transparent (PNG) renders on SwiftShader (Docker) " +
+        "auto-fall back to screenshot capture. Incompatible with page-side " +
+        "shader compositing. Default: false. Env: PRODUCER_EXPERIMENTAL_FAST_CAPTURE.",
+      // No `default` — an omitted flag must stay `undefined` so the `!= null`
+      // guard below leaves PRODUCER_EXPERIMENTAL_FAST_CAPTURE untouched and the
+      // env fallback survives (matches the --low-memory-mode idiom).
+    },
   },
   // `run` is the citty handler for `hyperframes render` — sequential flag
   // validation + render dispatch. Inherited CRITICAL on main (CRAP 1290);
@@ -513,6 +525,13 @@ export default defineCommand({
       process.env.PRODUCER_LOW_MEMORY_MODE = args["low-memory-mode"] ? "true" : "false";
     }
 
+    // ── Override: experimental fast capture (drawElementImage) ───────────
+    if (args["experimental-fast-capture"] != null) {
+      process.env.PRODUCER_EXPERIMENTAL_FAST_CAPTURE = args["experimental-fast-capture"]
+        ? "true"
+        : "false";
+    }
+
     // ── Validate max-concurrent-renders ─────────────────────────────────
     if (args["max-concurrent-renders"] != null) {
       const parsed = parseInt(args["max-concurrent-renders"], 10);
@@ -915,6 +934,7 @@ export default defineCommand({
         entryFile,
         outputResolution,
         pageSideCompositing: args["page-side-compositing"] !== false,
+        experimentalFastCapture: args["experimental-fast-capture"] === true,
         pageNavigationTimeoutMs,
         protocolTimeout,
         playerReadyTimeout,
@@ -984,6 +1004,8 @@ interface RenderOptions {
   /** Output resolution preset; see `resolveDeviceScaleFactor` for constraints. */
   outputResolution?: CanvasResolution;
   pageSideCompositing?: boolean;
+  /** EXPERIMENTAL. drawElementImage frame capture (--experimental-fast-capture). */
+  experimentalFastCapture?: boolean;
   /**
    * Puppeteer `page.goto()` timeout for the entry HTML, in milliseconds.
    * When omitted, the engine default (60s) applies. Surfaced as
@@ -1293,6 +1315,7 @@ async function renderDocker(
       outputResolution: options.outputResolution,
       pageSideCompositing: options.pageSideCompositing,
       debug: options.debug,
+      experimentalFastCapture: options.experimentalFastCapture,
       pageNavigationTimeoutMs: options.pageNavigationTimeoutMs,
     },
   });