tools: marketing screenshot and demo-video capture

tools/marketing-capture/README.md

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+# Marketing capture
+
+Produces screenshots and demo videos of the renderer UI for the marketing site,
+without running Electron, logging in, or holding a real interview.
+
+```bash
+pnpm dev                                              # renderer on :15173
+python tools/marketing-capture/capture.py shots  out/
+python tools/marketing-capture/capture.py videos out/ # needs ffmpeg on PATH
+```
+
+Requires Python with `playwright` and its Chromium browser
+(`pip install playwright && python -m playwright install chromium`).
+Override the URL with `CAPTURE_URL` if the dev server is elsewhere.
+
+## How it works
+
+The renderer reaches the main process through exactly one object -
+`window.electron`, exposed by [src/main/preload.cts](../../src/main/preload.cts).
+It boots fine in an ordinary browser and then waits on that object, so the
+script injects a stub for it via `addInitScript` and the real UI renders
+against canned state. No Electron, no backend, no audio, no credentials.
+
+Video mode uses the same stub but keeps the subscriber list live and mutates
+`STATE` in place over a timeline, so the panels animate the way they do in a
+real session: transcript lines arrive, a card goes pending, the answer streams
+in, then a screenshot solution lands.
+
+## Why it lives in this repo
+
+Everything it mirrors is here - the preload surface, `RendererAppState`, the
+`RuntimeConfig` defaults, the `Speaker` / `SuggestionState` / `RunningState`
+enum values. When any of those change this is what has to change with them, and
+it should break next to the code that broke it. Same reasoning as `test/manual/`.
+
+It is deliberately **not** in `test/`: it asserts nothing and CI does not run it.
+
+## Three things that are easy to get wrong
+
+- **`zoom.getFactor()` returns a multiplier, not a percentage.** The control
+  renders `factor * 100`, so returning `100` puts `10000%` on the status bar.
+- **`audioInputDeviceName` has to match a device the browser really
+  enumerates.** Headless Chromium has none, so the control bar raises its
+  "configured microphone is missing" badge on every frame. The script launches
+  with `--use-fake-device-for-media-stream` and points the config at
+  `Fake Default Audio Input`.
+- **Stealth is a body class, not only app state.** The preload adds
+  `.stealth` to `document.body` on `window:stealth-changed`; setting
+  `isStealth` alone leaves the normal chrome on screen.
+
+## Demo content
+
+Illustrative, not a recording of a real session: a generic backend-engineering
+question about indexing, and a connected-components solution. No real names,
+companies, or identifiable details.
+
+**Do not paste a real candidate's transcript in here.** These images go on a
+public marketing page.
+
+## What it does not cover
+
+The **stealth overlay composited over a real screen share** - the shot that
+actually proves the feature - cannot come from this script, because there is no
+desktop behind the window to be invisible against. That one needs the real app:
+
+```bash
+pnpm electron:dev-show -- --disable-content-protection
+```
+
+Content protection is on by default, so an ordinary screen recorder captures
+the stealth window as blank; that flag
+([src/main/index.ts](../../src/main/index.ts)) is what makes a real screen
+recording of it possible at all.