ci: decouple codspeed from ci gate

[Mason Daugherty (mdrxy)]

CodSpeed benchmarks shared the main CI workflow's concurrency group, which has cancel-in-progress: true. On push-to-master (post-merge), back-to-back merges would cancel in-flight CodSpeed jobs — failing the ci_success gate and marking the merge commit red, even though the PR passed all checks before merging. The cancelled jobs also meant baseline benchmark data was never uploaded, silently breaking CodSpeed's regression detection for subsequent PRs.

Changes

• Extract CodSpeed into a standalone codspeed.yml workflow with its own concurrency group — push-to-master runs use github.sha as the group key (unique per commit, never cancelled), while PR runs continue to cancel stale runs via github.ref
• Remove the codspeed job, its output, and its ci_success gate dependency from check_diffs.yml — benchmark results are informational and shouldn't block merges

Review

• The new workflow duplicates the build (change detection) job from check_diffs.yml since it can't reference jobs across workflows. This means an extra check_diff.py run per trigger, but it's lightweight (~2s) and the tradeoff is worth the isolation.