Wrap whisper.cpp's built-in Silero VAD

[uqio]

Tracked under #6.

Scope

Wrap whisper.cpp's bundled Silero ONNX VAD so callers who don't already have a VAD pipeline can opt into the built-in one without dropping to FFI.

Symbols

• whisper_vad_* family (init / detect / free).
• whisper_full_params::vad, vad_model_path, vad_params.
• set_min_speech_duration_ms, set_max_speech_duration_s, set_min_silence_duration_ms, set_speech_pad_ms, set_threshold.

Why deferred

Whispery uses the silero crate upstream — re-wrapping whisper.cpp's bundled VAD duplicates state and gives whispery two VAD pipelines to keep in sync. The built-in path is useful for callers who don't already have a Rust-side VAD; whispery isn't one of them.

Acceptance

• New Context::detect_vad(...) (or similar) entry point for whisper.cpp's whisper_vad_* flow, with the parameter setters as WhisperVadParams::with_*.
• Patch verification entries in whispercpp-sys/build.rs if any new submodule sentinels are needed.
• Add an axis-matrix row in safety_audit.rs covering the new method.
• Tests on a poisoned fixture (no model required) plus a feature-gated integration test if a small fixture model is available.

If you're a caller who needs this, drop a comment with your scenario.