Add cross-platform audio backend foundation

[pranc1ngpegasus]

Summary

• Add backend-neutral capture and transcript contracts in wisp-core.
• Add capability-aware, privacy-preserving backend selection and bounded session orchestration in wisp-audiokit.
• Harden Windows WASAPI recording, lifecycle, timeline, overflow, and callback paths while preserving macOS and legacy compatibility.

Scope

This PR establishes the cross-platform abstraction foundation. The Linux PipeWire adapter, the Windows local-model inference connection, and a Nemotron backend remain follow-up work. Existing SessionConfig, SourceLabel, and macOS WispAudioKit compatibility are retained.

Validation

• cargo test --workspace --exclude wisp-desktop --no-fail-fast (wisp-audiokit 83 tests, wisp-core 7 tests, plus workspace suites and doctests)
• cargo clippy --workspace --exclude wisp-desktop --all-targets -- -D warnings
• cargo fmt --all -- --check
• bash formal/check.sh (Kani 5/5, Shuttle 2/2)
• Windows MSVC xwin Clippy with tests and warnings denied
• Final implementation, test, and security reviews: clean

Limitations

• The full desktop workspace build is blocked in this local environment because the Apple metal compiler is unavailable.
• Windows-specific paths were cross-compiled and linted, but were not exercised on physical Windows hardware.