feat: build native menu-bar shell and dictation state machine

[douglasjarquin]

Closes #3

Summary

• Adds the native macOS 26+ arm64 AppKit menu-bar shell with NSStatusItem, accessory activation, no Dock icon, configurable Carbon global toggle, and a nonactivating status surface.
• Adds the explicit dictation coordinator state machine, legal/illegal transition handling, cancellation and shutdown cleanup, unified logging/signposts, and focused contract coverage.
• Adds the production Xcode project metadata, SwiftPM build target, idle policy documentation, and CI contract harness coverage without adding runtime dependencies.

Validation

• swift build
• swift build --product Oigo
• swift run oigo-issue3-contract-tests
• swift run oigo-spike-contract-tests
• plutil -lint Oigo.xcodeproj/project.pbxproj
• plutil -lint Oigo/Info.plist
• git diff --check
• arm64 binary and system-only linkage checks

Real-surface evidence is captured in .ulw-issue-3-evidence/desktop-qa.md, .ulw-issue-3-evidence/idle-baseline.md, and .ulw-issue-3-evidence/validation.md.

Limitations

• The host has Command Line Tools but not full Xcode, so xcodebuild and the Xcode project build could not be run here.
• The documented short idle baseline passed at 0.0% CPU and 11-12 MB across seven five-second samples.
• The required 30-minute Xcode/Instruments idle measurement remains unverified for the same host limitation.