ci: parallelize validation and optimize release workflows

[entelecheia]

Summary

• parallelize pull-request CI through a lightweight decision job and independent make verify / Playwright E2E jobs
• parallelize Release Preflight into three independent jobs: make release-preflight-core, release-mode make cli-smoke, and Playwright E2E
• add the reusable Makefile core target while keeping the local aggregate preflight complete
• install only the active macOS matrix target with rustup in release bundle jobs
• update README documentation for the optimized CI and release flow

Current revision

This draft contains 4 commits and currently points to cce61ed19934a3971e6c12b19b5d3c147a074ab7:

1. ci: parallelize pull request validation
2. ci: parallelize release preflight
3. ci(release): narrow macOS targets and document parallel checks
4. ci: parallelize release CLI smoke

Live workflow evidence

• PR CI run 32737230925: success in 5m55s
  • make verify: 5m06s
  • playwright e2e: 5m48s
  • both validation jobs ran in parallel after the decision job
• Release Preflight run 32737304599: success in 10m37s
  • make release-preflight-core: 10m32s
  • make cli-smoke: 8m12s
  • playwright e2e: 6m21s
  • all three preflight jobs ran in parallel

Local validation

• make verify: 1,858 TypeScript tests; 1,219 Rust tests passed with 3 ignored
• make test-e2e: 203 passed
• actionlint passed
• workflow YAML parsing passed
• git diff --check passed

Release safety

• validation coverage remains intact; the optimization changes scheduling and redundant setup rather than skipping checks
• the local aggregate release preflight still covers core validation, release CLI smoke, and E2E
• release bundle topology, signing, notarization, updater, publishing, and Homebrew integrity gates remain unchanged