Implement realtime dictation HUD and model settings

[yvette-carlisle]

Summary

• add Rust-owned Realtime Pass1 dictation using gpt-realtime-2 session config, transcript assembly, and Pass2 fallback
• add macOS global hotkey handling and non-activating floating recording HUD
• expose realtime/finalize/rewrite model choices in Settings and persist them through Rust config
• update runtime docs/runbook for the new Settings-backed behavior

Verification

• cargo make fmt
• cargo make checks
• semantic drift audit pass