ci(nix): record flake, build, and smoke timings in job summaries

[davidabram]

Pull request stack

Review and merge bottom-up. Each PR targets the previous PR's branch, so Files changed shows only that step's incremental changes.

Step	Pull request	Change
1/4	#308 — Faster Nix checks	Rust test-profile cache; deduplicate default MBT check
2/4	#309 — Quint workflow cleanup	Stop recompiling Rust MBT separately
3/4	#310 — macOS smoke-build reuse	Reuse release artifact for macOS smoke tests
4/4	#311 — CI timing summaries	Record Nix phase durations in job summaries

You are reviewing step 4/4: #311. Parent: #310. Next: #315 — Crane test dependency preparation. Four follow-up PRs continue through #318.

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Measure each successful Nix phase with Bash SECONDS and print its duration to the GitHub Actions job summary: flake checks, Linux native build/smoke, per-platform release validation, and macOS release smoke. This makes future cold/warm CI comparisons discoverable without downloading large logs, while preserving the original commands and failure behavior (run steps already use shell fail-fast).

Validation: workflow lint via nix flake check and Linux/macOS GitHub Actions. Recording uses integer-second precision, not a performance benchmark by itself.