[Tier 2] Expose FluidAudio diarizer tuning knobs (clustering threshold / min duration / numSpeakers hint)

[fmasi]

Symptom: Too many speaker boxes; no control over clustering behavior.

Root cause

TranscriberCore/FluidAudioDiarizer.swift:88 passes only OfflineDiarizerConfig(embeddingExcludeOverlap: false) and numSpeakers: nil (:18). No clustering threshold, min/max speakers, or min-duration is exposed; the app relies entirely on SDK defaults.

Proposed fix

• Investigate what FluidAudio 0.14.4 OfflineDiarizerConfig / process() actually expose (clustering threshold, min active duration, numSpeakers hint).
• Surface the useful ones via Config with sane defaults.
• Marked research because the available knobs must be confirmed against the SDK first.

Acceptance criteria

• SDK configuration surface documented.
• Useful knobs wired through Config with defaults.