ci(rust): run the Rust workspace, which no workflow has ever built

[EtienneLescot]

The gap

No workflow invokes cargo. crates/compositor is production code the app links, crates/compositor-view-napi is the addon it ships, and neither has ever been compiled by CI. The 76 unit tests in the workspace run only when someone remembers to run them locally, and nothing catches a crate that stops building.

This surfaced reviewing #161, which added unit tests as an acceptance criterion — tests that no automated system would ever have run.

Why it wasn't just "add a job"

A runner cannot compile the workspace as things stand. FFMPEG_DIR in crates/.cargo/config.toml points at a gitignored tree: bindgen needs its include/, the linker needs its lib/, and nothing has ever provisioned either. The steps were folklore — hand-place the archive, or junction poc-d3d/thirdparty and hope.

So fetch-ffmpeg.mjs now keeps that tree. It already downloads the shared build, verifies its pinned SHA-256, and asserts the licence is LGPL — then extracted include/ and lib/ and deleted them with the temp dir, keeping only the DLLs. Landing them at FFMPEG_DIR reuses the existing pin, digest and licence check instead of adding a second download path to keep in sync, and it fixes the clean-checkout build for contributors too, not just for CI.

Two details worth a look in review:

• The destination is parsed out of the cargo config, the same way pinnedFfmpegDir() in compositorViewService.ts reads it for the runtime PATH — a rename can't silently desynchronise them.
• It copies over the tree rather than replacing it. crates/thirdparty is commonly a junction into poc-d3d/thirdparty, and a recursive delete would follow the junction and take the real tree with it.

The job

windows-latest by necessity — the compositor is D3D11 and links the windows crate, so there is nothing to run on ubuntu.

• Runs from crates/ rather than passing --manifest-path: cargo resolves .cargo/config.toml against the working directory, and from the root the build gets neither FFMPEG_DIR nor LIBCLANG_PATH.
• --workspace because default-members = ["poc-d3d"] means a bare cargo test skips the library and the addon.
• --all-targets because it excludes doctests, and it has to: bindgen copies ffmpeg's C doxygen comments verbatim into out/ffi.rs, so rustdoc finds 13 "examples" that are C and fails to compile them as Rust. They are generated, and there is nothing in them to fix.

Deliberately not included

• cargo fmt --check fails today on files nobody touched here (audio.rs among them).
• clippy on FFI-heavy code, against a generated binding that already emits 13 warnings, is a backlog rather than a gate.

Both are ratchets worth adding once someone has cleared them — the same shape as the existing typecheck-tests job. The gate that pays for itself now is "it builds and the tests pass".

Verification

Locally, with the junction removed so the runner's cold state was reproduced: fetch-ffmpeg.mjs materialised the tree at crates/thirdparty, cargo clean -p openscreen-compositor forced bindgen to re-run against it, and cargo test --workspace --all-targets passed 76/76. Re-running the script is a no-op.

Beyond that, this PR verifies itself — the job below is the change.

Expect the first run to be slow (cold Swatinem/rust-cache, and Windows runners are not fast); subsequent runs hit the cache.

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