## Linked issue
Closes#4839
## Summary / motivation
Adds `cargo-llvm-cov`-based test coverage for the full Rust workspace.
Introduces `just test-rust --coverage` and `just test-rust --coverage
--html`, and wires Rust into the `just test --coverage` umbrella.
`cargo-llvm-cov` is installed on demand into `out/bin/` to avoid
polluting the global cargo install. The `llvm-tools-preview` rustup
component is now installed in CI so the tool can instrument binaries.
## How to test (required)
```sh
# Existing behavior unchanged
just test-rust
# Terminal summary
just test-rust --coverage
# Terminal summary + HTML report under out/coverage/rust/html/
just test-rust --coverage --html
# Umbrella (Rust + Python)
just test --coverage
just test --coverage --html
```
Note: first run of `--coverage` will install `cargo-llvm-cov` into
`out/bin/` (~30s). Subsequent runs skip the install step.
### Checklist
- [x] I ran `./ninja check` or an equivalent relevant check locally.
### Details
- `cargo-llvm-cov` pinned at `0.8.4`, installed into `out/bin/` via
`cargo install --root out`.
- `--workspace --locked` measures all crates and respects the lockfile.
- `llvm-tools-preview` added to `setup-anki` action so CI can instrument
Rust binaries.
- Coverage runs are slower than plain `just test-rust` because
`cargo-llvm-cov` rebuilds with instrumentation — this is expected.
## Before / after behavior
Before: no `just test-rust`, no Rust coverage support.
After: `just test-rust` runs Rust tests via ninja; `just test-rust
--coverage` runs them with `cargo-llvm-cov`
---------
Co-authored-by: Abdo <abdo@abdnh.net>
