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/utils/dockerRunArgs.test.ts

@@ -173,6 +173,7 @@ describe("buildDockerRunArgs", () => {
         quiet: true,
         debug: true,
         entryFile: "compositions/intro.html",
+        experimentalFastCapture: true,
       },
     });
     // Each value must reach the container exactly once. If a future option
@@ -195,6 +196,24 @@ describe("buildDockerRunArgs", () => {
     expect(args).toContain("--hdr");
     expect(args).toContain("--composition");
     expect(args).toContain("compositions/intro.html");
+    expect(args).toContain("--experimental-fast-capture");
+  });
+
+  it("forwards --experimental-fast-capture only when enabled", () => {
+    const on = buildDockerRunArgs({
+      ...FIXED_INPUT,
+      options: { ...BASE, experimentalFastCapture: true },
+    });
+    expect(on).toContain("--experimental-fast-capture");
+
+    const off = buildDockerRunArgs({
+      ...FIXED_INPUT,
+      options: { ...BASE, experimentalFastCapture: false },
+    });
+    expect(off).not.toContain("--experimental-fast-capture");
+
+    const absent = buildDockerRunArgs({ ...FIXED_INPUT, options: BASE });
+    expect(absent).not.toContain("--experimental-fast-capture");
   });
 
   it("forwards --format png-sequence to the container", () => {

packages/cli/src/utils/dockerRunArgs.ts

@@ -56,6 +56,8 @@ export interface DockerRenderOptions {
   /** Output resolution preset (e.g. "landscape-4k"). Forwarded as `--resolution`. */
   outputResolution?: string;
   pageSideCompositing?: boolean;
+  /** EXPERIMENTAL. drawElementImage frame capture; forwarded as `--experimental-fast-capture`. */
+  experimentalFastCapture?: boolean;
   /**
    * Puppeteer page-navigation timeout, in milliseconds. Forwarded to the
    * in-container CLI as `--browser-timeout <seconds>` (the CLI takes
@@ -144,6 +146,7 @@ export function buildDockerRunArgs(input: DockerRunArgsInput): string[] {
     ...(options.entryFile ? ["--composition", options.entryFile] : []),
     ...(options.outputResolution ? ["--resolution", options.outputResolution] : []),
     ...(options.pageSideCompositing === false ? ["--no-page-side-compositing"] : []),
+    ...(options.experimentalFastCapture ? ["--experimental-fast-capture"] : []),
     ...(options.pageNavigationTimeoutMs != null
       ? ["--browser-timeout", String(options.pageNavigationTimeoutMs / 1000)]
       : []),