Complete macOS audio backend migration

[pranc1ngpegasus]

Summary

• Migrate macOS desktop and legacy session entry points onto the backend-neutral SessionOrchestrator with production MacosCaptureBackend and MacosTranscriberBackend implementations.
• Route Core Audio / Process Tap PCM through a bounded Rust handoff into SpeechAnalyzer via TranscriberBackend::push, while keeping Ogg recording on the capture side.
• Split capture/transcriber lifecycle operations, add record-only fallback and strict-policy behavior, and propagate runtime failures, overflow, gaps, and timestamps deterministically.
• Preserve/version the v1/v2 C ABI and expand Rust, Swift, desktop, lifecycle, and cross-target contract coverage.

Why

PR #108 introduced the cross-platform capture/transcription abstractions, but macOS production code still had paths that bypassed the orchestrator. This completes that migration so macOS uses the abstraction end to end before Windows/Linux backends are added.

Base

• PR Add cross-platform audio backend foundation #108 has been merged.
• This PR now targets main directly.

Impact

• macOS capture, transcription, fallback, shutdown, and failure semantics now live behind the shared backend contracts.
• Existing v1 clients remain compatible; new orchestration behavior is exposed through the versioned v2 ABI.
• No Windows/Linux production backend is added in this PR.

Validation

• cargo test --workspace --exclude wisp-desktop --no-fail-fast
• cargo clippy --workspace --exclude wisp-desktop --all-targets -- -D warnings
• cargo test -p wisp-desktop --features gpui/runtime_shaders session_runner::tests (2 passed)
• nix shell nixpkgs#cargo-xwin --command env RUSTC_BOOTSTRAP=1 cargo xwin clippy -Z build-std=std,panic_abort -p wisp-audiokit --target x86_64-pc-windows-msvc --tests -- -D warnings
• swift build --package-path native/WispAudioKit
• bash formal/check.sh (Kani 5/5, Shuttle 2/2)
• cargo fmt --all -- --check
• git diff --check
• Six ren review workflows (57/96 slots); final independent review returned clean.

Environment-only gaps

• Swift XCTest cannot run in the local Command Line Tools image because XCTest/xctest is unavailable; the Swift product itself builds successfully.
• A normal local desktop build requires Apple metal; the runtime-shader test path compiled and exercised the affected session runner.
• Physical microphone / Process Tap / TCC / SpeechAnalyzer end-to-end behavior still requires macOS hardware validation.