docs: make the README a product page, with generated screenshots

[smb]

Closes #43.

Stacked on #47. The base is fix/english-provider-labels, because the
screenshots have to show the English provider labels that #47 introduces.
GitHub will retarget this to main when #47 merges.

What changed

The README leads with the product. What MatteLoop does, what comes out, and
that nothing leaves the machine — all before the first command. The developer
half survives intact further down: running from source, the check commands, the
diagnostic flags, the cache-migration note, and the pointer to
docs/building.md. The licence section is untouched.

rembg is credited in the body, by name and link, with a sentence saying what
is used from it — the catalog, the session classes, the approach, and the
weights. The individual model architecture authors stay in
THIRD_PARTY_NOTICES.md, pointed to rather than repeated.

Three screenshots, generated rather than collected. scripts/screenshots.py
decodes assets/demo/golden-retriever.mp4 with the application's own PyAV path,
segments it with the pinned rembg model, and captures real Qt widgets. The
README documents how to regenerate them, so a layout change cannot leave them
quietly wrong.

The fixture-rotation contract moved to docs/testing-fixtures.md, where the
people who need it are, with nothing lost.

What the script drives for real, and what it seeds

Real: the decoded source frame, the segmentation cutout, every widget, the model
catalog, and the timeline facts derived from the clip.

Seeded: the images reach the window through an immutable state snapshot rather
than by starting the asynchronous preview job; the output directory is a neutral
demo path; and the model dialog is pointed at a throwaway cache root holding one
sparse placeholder, so the picture is identical on every machine instead of
reflecting whoever ran it last. The script's own docstring says the same thing,
so nobody later mistakes it for an end-to-end test.

The script touches no application code — git diff against the README commit
shows nothing under src/ or tests/. It is not imported by the test suite and
does not run during it.

Test plan

• uv run ruff check ., uv run mypy src, QT_QPA_PLATFORM=offscreen uv run pytest -q and scripts/check_guardrails.py all pass.
• Running the capture script twice from clean produces byte-identical screenshots.
• The real user model cache is neither read nor written by the capture.
• Every link the README makes resolves to a file that exists.

🤖 Generated with Claude Code