ci: run independent static checks in parallel

[NachoooLK]

What Changed

Run identity, Windows runtime boundary, format, lint and release smoke concurrently with GitHub Actions' native parallel group in static-fast. Workspace setup and CI contract validation run first. The group waits for all five checks and propagates failures to the existing required gate, while retaining separate step logs.

Document the shared workspace and temporary release-smoke fixture boundaries in .github/CI.md.

Why

These independent checks currently run sequentially on an already installed runner. Native parallel steps allow them to overlap without another checkout or install. Test/build runner partitioning and every check command remain unchanged. No overall CI speedup is claimed without hosted measurements.

Verification

Based on upstream/main 24365fb5eb8648dd1b597760acaaa43ea3ec495d.

• bun run fmt:check, bun run lint, bun run typecheck, bun run test
• node --test .github/scripts/ci-contracts.test.mjs (14 tests)
• bun run brand:check, bun run windows-runtime:check, bun run migrations:check
• node scripts/release-smoke.ts
• Parsed YAML comparison confirms that flattening the group preserves all original job settings and steps exactly. Actionlint 1.7.12 validates the flattened workflow, but does not yet support the native parallel keyword; actual native execution is covered by this PR's Actions run.
• Hosted native parallel execution passed: all five checks started at 23:10:45 UTC, and the group completed at 23:10:56 UTC. This is one observed group duration, not a before/after benchmark or overall CI saving.
• Full PR CI completed successfully; all 23 PR checks passed, including the required aggregate, six browser partitions, three server shards, native Windows regression and desktop Electron checks.

Checklist

• This PR is small and focused
• I explained what changed and why