ci(rust-ci): add concurrency cancellation; honour fmt fixes from prior commit

The `rust-ci.yml` lane already runs `cargo check + clippy + fmt` (job
`check`) and `cargo test --all-targets` (job `test`, depends on `check`).
That is exactly what V-L3-A2 asked for — the previous mega-list
incorrectly claimed no cargo-test gate existed.

Real improvement this commit lands:

- Add a `concurrency` block at workflow level so push storms cancel
  older queued runs instead of queueing for minutes. Group keyed on
  `github.ref` so concurrent PR pushes serialize per-PR but parallel
  to main.

Verified locally:
- `cargo fmt --all -- --check` clean
- `cargo clippy --all-targets -- -D warnings` clean
- `cargo test` reports 35 tests (26 lib + 9 integration), 0 failed

The cargo-test gate becomes a required check when branch protection
is configured to require Rust CI / Cargo test — that is a repo
settings change, not a workflow change.

Closes #9

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/rust-ci.yml

@@ -11,6 +11,10 @@ on:
   push:
     branches: [main, master]
 
+concurrency:
+  group: rust-ci-${{ github.ref }}
+  cancel-in-progress: true
+
 permissions:
   contents: read