test(producer): add long-form render fixture generator and manual gate notes

The §8 fixtures every phase's manual gate renders against: one 300 s video
spanning the whole composition, and the same source cut into 60 clips.
Committed as a generator rather than fixtures because the source alone is
~600 MB.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/producer/tests/long-form/README.md

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+# Long-form render fixtures
+
+Manual gates for `plans/long-form-render/`. Not part of the automated lanes (they render for minutes and need ~600 MB of source).
+
+## Generate the source once
+
+```sh
+mkdir -p /tmp/hf-longform/assets && cd /tmp/hf-longform
+ffmpeg -y -f lavfi -i "testsrc2=size=1920x1080:rate=30" -f lavfi -i "sine=frequency=440:sample_rate=48000" \
+  -t 300 -c:v libx264 -preset ultrafast -crf 23 -g 30 -pix_fmt yuv420p -c:a aac -b:a 96k -movflags +faststart assets/long.mp4
+node <repo>/packages/producer/tests/long-form/gen.mjs
+```
+
+For the 40-minute soak fixture use `-t 2400` and `DUR=2400 node .../gen.mjs`.
+
+## Run a gate against the worktree build
+
+```sh
+cd <repo> && bun run build
+cd /tmp/hf-longform
+node <repo>/packages/cli/dist/cli.js render a-single --fps 30 -w 1 --quality draft -o a-single/renders/out.mp4
+```
+
+Check duration: `ffprobe -v error -show_entries format=duration -of csv=p=0 a-single/renders/out.mp4` → `300.000000`.
+
+Never run two of these concurrently on a dev Mac — the fleet limit is why the gates are sequential.
+
+## Gates by phase
+
+| Phase | Command                                                   | Expect                                                                                          |
+| ----- | --------------------------------------------------------- | ----------------------------------------------------------------------------------------------- |
+| 0     | `render a-single --fps 30 -w 1` (stock env)               | log `streaming-encode gate {"enabled":true,"reason":"single_worker",...}`; output 300.000 s     |
+| 1     | `render a-single --fps 30 -w 4` (stock env)               | log `Parallel screenshot capture will stream to the encoder`; work dir < 2 GB; output 300.000 s |
+| 2a    | `HF_SEGMENTED_CAPTURE=true render a-single --fps 30 -w 1` | `segment_*.mp4` in work dir; output 300.000 s; PSNR ≥ 45 dB vs the Phase 0 output               |
+| 2b    | kill the 2a render at ~40 %, rerun with `--resume`        | log `resuming: N segments complete`; output byte-identical to an uninterrupted run              |
+| 2c    | 2a with `HF_SEGMENT_BROWSER_RECYCLE=1`                    | one `[Render] segment browser recycled` line per segment; output unchanged                      |
+| 2d    | 2a with `-w 4`                                            | four `segment worker` lines; output 300.000 s                                                   |
+
+The Phase 0 mutation check is the same render with `PRODUCER_STREAMING_ENCODE_DURATION_CAP_ENABLED=true`: the gate line must flip to `"enabled":false,"reason":"duration_cap"` and the render must fail at the disk preflight on a host without ~75 GB free.
+
+PSNR between two renders:
+
+```sh
+ffmpeg -i a.mp4 -i b.mp4 -lavfi "[0:v][1:v]psnr" -f null - 2>&1 | grep -o 'average:[0-9.inf]*'
+```

packages/producer/tests/long-form/gen.mjs

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+// Long-form render fixtures (spec §8). Run from a scratch directory that
+// already contains assets/long.mp4 (see README.md for the ffmpeg command).
+//   DUR=300 node gen.mjs        # 5-minute fixtures (default)
+//   DUR=2400 node gen.mjs       # 40-minute soak fixtures
+import { mkdirSync, writeFileSync, linkSync, existsSync } from "node:fs";
+
+const W = 1920;
+const H = 1080;
+const DUR = Number(process.env.DUR ?? "300");
+if (!Number.isFinite(DUR) || DUR <= 0)
+  throw new Error(`DUR must be a positive number, got ${process.env.DUR}`);
+if (!existsSync("assets/long.mp4")) throw new Error("assets/long.mp4 missing; see README.md");
+
+const comp = (
+  body,
+) => `<!doctype html><html><head><meta charset="utf-8"><style>html,body{margin:0;background:#000}#root{position:relative;width:${W}px;height:${H}px;overflow:hidden;background:#000}video{position:absolute;inset:0;width:100%;height:100%;object-fit:cover}#badge{position:absolute;right:40px;top:40px;font:700 64px sans-serif;color:#fff;background:rgba(0,0,0,.5);padding:8px 24px}</style></head><body>
+<div id="root" data-composition-id="root" data-width="${W}" data-height="${H}" data-start="0" data-duration="${DUR}" data-no-timeline>
+${body}
+<div id="badge">overlay</div>
+</div></body></html>`;
+
+const single = () =>
+  comp(
+    `<video class="clip" id="v0" src="assets/long.mp4" data-start="0" data-duration="${DUR}" data-media-start="0" data-has-audio="true"></video>`,
+  );
+
+const clips = (n = 60) =>
+  comp(
+    Array.from({ length: n }, (_, i) => {
+      const d = DUR / n;
+      return `<video class="clip" id="v${i}" src="assets/long.mp4" data-start="${(i * d).toFixed(3)}" data-duration="${d.toFixed(3)}" data-media-start="${(i * d).toFixed(3)}" data-has-audio="true"></video>`;
+    }).join("\n"),
+  );
+
+for (const [name, html] of Object.entries({ "a-single": single(), "c-clips60": clips() })) {
+  mkdirSync(`${name}/assets`, { recursive: true });
+  writeFileSync(`${name}/index.html`, html);
+  if (!existsSync(`${name}/assets/long.mp4`)) linkSync("assets/long.mp4", `${name}/assets/long.mp4`);
+  writeFileSync(
+    `${name}/hyperframes.json`,
+    JSON.stringify({ name, fps: 30, width: W, height: H }, null, 2),
+  );
+}
+console.log(`generated a-single c-clips60 (DUR=${DUR}s)`);