fix(engine): hide ffmpeg console windows on Windows - #3381

What

Pass windowsHide: true at every production ffmpeg/ffprobe spawn site.

Closes #3379.

Why

ffmpeg and ffprobe are console-subsystem binaries, and Node's child_process.spawn
defaults windowsHide to false. Every spawn therefore opens a visible console window on
Windows. A render shells out dozens of times across parallel workers, so the reporter saw a
burst of windows appear and disappear across their desktop, ~11 at once with 6 capture
workers.

Cosmetic, but it is the first impression of a render on Windows.

How

The report named two call sites. Seven production sites share the cause, so all seven
are fixed rather than the two mentioned:

windowsHide is a no-op on macOS and Linux, so it is applied unconditionally.

Left alone deliberately: parity-harness.ts, regression-harness.ts and
mediaTypeTestFixtures.ts all spawn ffmpeg too, but they are dev-only tooling that never
runs on a user's desktop. Changing them would widen the diff without changing the reported
behaviour.

Test plan

• Unit tests added/updated
• Manual testing performed
• Documentation updated (if applicable)

Regression test

runFfmpeg.windowsHide.test.ts mocks child_process and asserts on the options object
actually handed to spawn:

const options = spawnMock.mock.calls[0]?.[2] as { windowsHide?: boolean } | undefined;
expect(options?.windowsHide).toBe(true);

Asserted on behaviour rather than on source text, so a future call site that drops the flag
is caught by what it does, not by how it is written.

Verified it fails without the fix:

$ # windowsHide removed from runFfmpeg
FAIL  runFfmpeg.windowsHide.test.ts > hides the console window so Windows renders do not flash terminals
$ # restored
Tests  1 passed (1)

Other checks

• vitest packages/engine/src/utils/ — 651 passed, 20 files
• oxlint + oxfmt on all 7 touched files — clean
• Pre-commit hooks (fallow, typecheck, commitlint) — green

Before / after

I do not have a Windows machine, so I cannot capture the desktop screenshots this one
really wants. The observable change is at the spawn boundary, and that is what the test
pins:

before   spawn(getFfmpegBinary(), args)                          -> console window per call
after    spawn(getFfmpegBinary(), args, { windowsHide: true })   -> no window

The reporter's repro repo is linked in #3379 and is the right way to confirm the desktop
behaviour on a real Windows box before merge.