fix mic transcription after input device changes

[pranc1ngpegasus]

Summary

• observe AVAudioEngine configuration changes and restart the mic tap after input device switches
• reconfigure the mic transcription converter when the input format changes
• serialize mic engine teardown/reconfiguration to avoid races during stop

Verification

• swift build
• make -C native/WispAudioKit
• cargo fmt --check
• cargo build -p wisp-audiokit
• cargo test -p wisp-audiokit
• cargo clippy -p wisp-audiokit --all-targets

Note: full workspace cargo test/clippy was not used as the local environment lacks Xcode's metal tool required by gpui shader compilation.