ci: overhaul workflow with detect-changes, sccache, MSRV, and doc-tests

- Add detect-changes job with dorny/paths-filter to skip CI on non-code changes
- Add concurrency group with cancel-in-progress to avoid redundant runs
- Replace cross-platform OS matrix with single ubuntu-latest runner
- Add sccache via mozilla-actions/sccache-action to test and coverage jobs
- Add shared-key to rust-cache for cross-job cache reuse
- Add MSRV check job (Rust 1.88) with cargo check --locked
- Add cargo test --doc step to rustdoc job
- Switch coverage to cargo llvm-cov nextest for consistency with test job
- Fix ci-gate logic: check each job result explicitly instead of needs.*.result
- Add timeout-minutes to all jobs
- Set global RUSTFLAGS="-D warnings"; reset to "" in coverage job

Author: bug-ops

.github/workflows/ci.yml

@@ -149,6 +149,8 @@ jobs:
           shared-key: "ci"
       - name: Build docs
         run: cargo doc --no-deps --all-features
+      - name: Doc-tests
+        run: cargo test --doc --workspace --all-features
 
   security:
     name: Security Audit