ci: cache SwiftPM + compile once (faster build)

[karansinghgit]

Two low-risk speedups for the ~3-minute build job.

Changes

1. Cache resolved Swift packages (actions/cache, keyed on Package.resolved), resolved into a repo-local SourcePackages dir via -clonedSourcePackagesDirPath. Stops WhisperKit / FluidAudio / swift-transformers / DSWaveformImage from being re-fetched and recompiled every run.
2. Compile once. The old job ran a full build and a full test (which recompiled the app). Replaced with build-for-testing → test-without-building — a single compile, same signals (build gate + tests).
3. Preinstalled SwiftLint instead of brew install on every run (brew fallback kept).

Expected impact

Build job should drop from ~3 min toward ~1–1.5 min on cache hits (first run seeds the cache). Lint stays advisory and off the critical path.

Notes

• The self-test is this PR's own CI run: first run seeds the SwiftPM cache; a re-run should show the cache restored and a faster build.
• Bigger follow-up (not in this PR): a tag-triggered signed-release GitHub Actions workflow so releases don't require a local Mac — needs Developer ID + notary creds as repo secrets. Happy to scaffold it separately.