feat: add Linux runtime and offline voice

[jasonsuhari]

Summary

• add a Linux-only offline Whisper voice helper with native microphone capture
• pin and atomically install the multilingual base-q5_1 model on first approved use
• isolate ALSA/whisper dependencies from the core GridBash executable
• add Linux x64 CI for native PTY, helper, clippy, test, and release compilation

Current state

Draft while the shared Unix runtime and optional native-package framework from the coordinated macOS work are landing. The Linux branch will rebase onto those shared seams before Alt+V and release packaging are connected.

Validation

• cargo fmt --check
• cargo test (112 passed, 1 ignored)
• cargo clippy --all-targets -- -D warnings
• npm run test:launcher (8 passed)

Refs #141