feat: persist microphone input selection

[douglasjarquin]

Closes #76

Summary

• Add an event-driven local Core Audio input inventory with stable UID-backed selections.
• Expose System Default and available inputs in Settings and onboarding, including unavailable pinned recovery.
• Persist only the selected UID in local settings and preserve legacy settings during migration.
• Route the selected device before format inspection and tap installation, fail closed for missing pins, and fence capture callbacks and setup teardown.
• Add deterministic issue [V1 Phase 0 · P0] Add persisted microphone input selection #76 contract coverage and run it in CI.

Verification

• swift build
• swift build -c release --product Oigo
• swift run oigo-issue76-contract-tests
• Existing issue [02/13 · MVP] Build the native menu-bar shell and dictation state machine #3 through [09/13 · Reliability] Harden device changes, interruptions, cancellation, and failure paths #10 contract harnesses
• Native unsigned AppKit selector surfaces observed for onboarding and Settings, with clean quit

Native two-input, reconnect/disappearance, TCC decision, and Xcode validation remain inconclusive on this host because only one input is available, Xcode is not installed, and protected permission automation is not authorized. Those content-free native scenarios remain owned by #12. Stereo/channel conversion remains owned by #90.