fix(timeline): coalesce scrub state updates per frame - #520

…models and upstream fixes (#13)

Capturia 2.1: new editor and capture features, plus the upstream open PRs that were worth taking.

## New features
- **Record an area of the screen**: an Area tab in the source picker opens an overlay on the chosen display. You drag, move and resize a rectangle, and it shows the live size in physical pixels. The rectangle is validated and clamped in the main process, and the recording opens already cropped to it. Auto-zoom stays inside the area. Not offered on Wayland.
- **Saved looks**: save the current appearance (background and frame, camera layout, cursor, caption style, and optionally the format) as a named preset, apply it in one undo step, and star one as the default for new projects. Regions, trims, zooms, crop and the transcript are never touched.
- **Zooms at flagged moments**: Auto-enhance adds a zoom at every moment flagged while recording, using the same placement rules as auto-zoom. A flag that falls in a trim or on an existing zoom is reported, not duplicated.
- **Right-click menu** on region pills and clips: Copy, Paste at playhead, Split, Delete. These call the same functions as the keyboard shortcuts. Also fixes Ctrl+C on audio pills, which did nothing before.
- **Poster frames**: the project list and media cards show real thumbnails. They are generated by ffmpeg in the main process, cached, and made one at a time.
- **Speech model choice**: Fast / Balanced / Accurate in AI settings. Each model is pinned to a SHA-256 digest and verified before it becomes active, and a failed switch keeps the previous model.
- **Recordings folder**: choose where new takes are saved. The folder is set only through the OS picker. In that folder, only files Capturia itself names are reachable, after resolving symlinks, and it is never auto-cleaned. If the folder is unavailable, the app offers to use the default before the take starts.
- **Pre-release update channel**: opt-in, and it never downgrades (`allowDowngrade` stays false).

## Taken from upstream open PRs
Each one was rebuilt on our code where it no longer applied, and each carries its `Upstream-PR:` trailer:
getopenscreen/openscreen#302, #386, #519, #520, #571, #617, #632, #640, #641, #642, #644.
- #617 drops `node_modules` from `app.asar`. Verified: every npm dependency is bundled by Vite, since externals are Node builtins plus `electron`. `electron-updater` is a bundled chunk, and native addons load from `resourcesPath`.

## Fixes
- **Windows Store verify step**: it looked the package up by the pre-rename name, `EtienneLescot.OpenScreen`, which is what failed the RC.3 Store job. It now reads the name from the generated `AppxManifest.xml`.
- **Linux export on Intel Arc**: iHD accepts the dmabuf and then returns EIO on every encode, so every hardware export died at the first frame. Each export now probes one real frame and falls back to software if it fails. The mapped frame is also freed when `send_frame` fails.
- **Windows microphone drift**: the 44.1→48 kHz path rounded every packet on its own, which added up to 3.75 s/h of growing mic lag. It now carries the position across packets with exact integer totals. 88.2/176.4/352.8 kHz devices now snap to 44.1 kHz, so they go through the anti-alias decimator.
- **PipeWire test**: the vendored SPA 1.0.5 compares 64-bit values through an `int`, so the old probe modifier matched Intel X_TILED. The test now uses a modifier that cannot collide. CI now runs this crate's tests.

## Review and audit
The integrated branch got an independent security audit and a separate bug hunt. Both were read-only, and every finding was verified by tracing the code. Fixed here:
- **Self-update**: it could install a version other than the one the dialog named, or error out instead of falling back to "View Release". It now self-updates only when electron-updater's version matches.
- **Recordings folder**:
  - The writable check always passed on Windows, because libuv ignores directory ACLs. It now creates and deletes a real probe file.
  - Renderer-named writes are contained after resolving symlinks.
  - A take keeps the path it opened with, so changing the folder's availability mid-take no longer reports "missing on disk".
  - The folder cannot be changed while a take is running.
- **Poster cache**: one entry per source file, with no flicker when the duration arrives.
- **Speech models**: switching is single-flight, and a settings dialog reopened mid-download joins the running download.
- **Timeline**: a shift-click that deselects a pill no longer leaves it focused, which had made the menu delete the wrong pill.
- **Area recording**: a flag zoom with no telemetry now centres on the recorded area.
- **Saved looks**: applying a look is optimistic, so an edit made during its save is no longer lost.
- **Saved-looks probe document**: it was invalid at import time. Caught in review before it could crash the editor.

## Verification
- Both tsc projects exit 0. Biome is clean; the 26 warnings are the same as on main. The i18n check passes, with real translations in all 13 locales.
- Vitest: 255 files, 3099 passed, 1 skipped, on the integrated branch.
- Rust: compositor 216 lib tests plus integration tests, and pipewire-capture 84 tests. Both pass locally.
- C++ `audio_sample_utils_test`: 97/97 under g++ on Linux, using stub headers. MSVC coverage comes from the `build.yml` dispatch on this branch, which never publishes without `release_tag`.
- Every agent-reported claim was re-checked independently. For example, the model digests were checked against Hugging Face's LFS oids, and the electron-updater downgrade path was read in 6.8.9.

## Release note
The speech-model change adds a `--dtw-preset` flag to the whisper helper. An older helper ignores unknown flags, so Balanced keeps working. The 2.1 release must still be cut **after** `build-whisper-stt.yml` has finished on main, so the installers stage a helper that understands the flag.

Upstream-PR: getopenscreen/openscreen#302
Upstream-PR: getopenscreen/openscreen#386
Upstream-PR: getopenscreen/openscreen#519
Upstream-PR: getopenscreen/openscreen#520
Upstream-PR: getopenscreen/openscreen#571
Upstream-PR: getopenscreen/openscreen#617
Upstream-PR: getopenscreen/openscreen#632
Upstream-PR: getopenscreen/openscreen#640
Upstream-PR: getopenscreen/openscreen#641
Upstream-PR: getopenscreen/openscreen#642
Upstream-PR: getopenscreen/openscreen#644