[TUNER88]

Recovers #20, includes #21/#22.

#20 was closed unmerged when its base branch data/sounds-json was deleted after #18 merged. #21 (DocC) and #22 (SoundBrowser) were merged into feature/swift-package, not into master. This PR cherry-picks the same four commits onto current master. The files are byte-identical to feature/swift-package.

Why it's one PR: the DocC catalog lives inside the package target, and the example app links the package through a local reference. Neither builds on master without the package. Separate PRs would have to be stacked again, which is what caused the problem.

Contents, one commit each:

1. Swift package (from Add Swift package SystemSoundsLibrary (Swift 6, generated from data/sounds.json) #20):
   • Package.swift (swift-tools 6.0; iOS 15, tvOS 15, visionOS 1, Mac Catalyst 15, macOS 12)
   • SystemSound (Sendable, 418 constants generated from data/sounds.json, @available on newer IDs, screenUnlocked deprecated)
   • SystemSoundPlayer, plus FileSound, which frees its sound ID automatically
   • Swift Testing tests
   • swift.yml: runs swift build + swift test, checks the generated catalog with generate_swift.py --check, and builds for 4 platforms
2. DocC catalog and docc.yml (from Add DocC documentation for SystemSoundsLibrary #21): docc convert --warnings-as-errors, plus an output check (both commits).
3. Examples/SoundBrowser (from Add SoundBrowser SwiftUI example app #22): SwiftUI iOS 17+, searchable, grouped by folder, tap to play, shows ID and path, share or save the file. example.yml builds it for the iOS Simulator. Refs If possible to export all the sounds? #2, Swift version? #7.

The existing Objective-C demo app, its project, target, paths and README content are unchanged. The must-not-break check passes.