feat(cli): expose png-sequence format

The producer already supports `format: "png-sequence"` end-to-end (see
RenderConfig in renderOrchestrator.ts), but the CLI's VALID_FORMAT
validator rejects it before the flag reaches the producer. Surface it
the same way `mov` and `webm` are surfaced.

Behaviour:
- `--format png-sequence` accepted alongside mp4/webm/mov.
- Auto-output path uses no extension (FORMAT_EXT["png-sequence"] = "")
  since the producer treats outputPath as a directory of frame_NNNNNN.png.
- `printRenderComplete` sums the contained file sizes when outputPath
  is a directory, instead of reporting the platform-dependent inode
  size.
- DockerRenderOptions.format type extended; existing buildDockerRunArgs
  is unchanged because it forwards the string verbatim.

Tests:
- renderLocal forwards `format: "png-sequence"` to createRenderJob.
- buildDockerRunArgs propagates `--format png-sequence` to the
  container.

Docs:
- Rendering guide: format flag table, format comparison table, new
  "PNG sequence (no encoding)" section, "How it works" extended.
- CLI package docs: format flag table updated.

packages/cli/src/utils/dockerRunArgs.test.ts

@@ -181,6 +181,17 @@ describe("buildDockerRunArgs", () => {
     expect(args).toContain("compositions/intro.html");
   });
 
+  it("forwards --format png-sequence to the container", () => {
+    const args = buildDockerRunArgs({
+      ...FIXED_INPUT,
+      outputFilename: "frames",
+      options: { ...BASE, format: "png-sequence" },
+    });
+    const formatIdx = args.indexOf("--format");
+    expect(formatIdx).toBeGreaterThanOrEqual(0);
+    expect(args[formatIdx + 1]).toBe("png-sequence");
+  });
+
   it("forwards --video-bitrate to the container when set", () => {
     const args = buildDockerRunArgs({
       ...FIXED_INPUT,

packages/cli/src/utils/dockerRunArgs.ts

@@ -21,7 +21,7 @@ export interface DockerRunArgsInput {
 export interface DockerRenderOptions {
   fps: 24 | 30 | 60;
   quality: "draft" | "standard" | "high";
-  format: "mp4" | "webm" | "mov";
+  format: "mp4" | "webm" | "mov" | "png-sequence";
   workers?: number;
   gpu: boolean;
   browserGpu: boolean;