Add SpeakerKit with Pyannote speaker diarization support (#440)

On-device speaker diarization using the [Pyannote v4 (community-1)](https://huggingface.co/argmaxinc/speakerkit-coreml) pipeline.

```
swift run whisperkit-cli diarize --audio-path audio.wav [--verbose] \
  [--num-speakers N] [--cluster-distance-threshold F] \
  [--model-path /path] [--model-repo org/repo] [--download-model-path /path] \
  [--rttm-path output.rttm]
```
Output is standard RTTM. Use --model-path for existing models; otherwise models are resolved/downloaded as needed (e.g. via --download-model-path).

```
swift run whisperkit-cli transcribe --audio-path audio.wav --diarization [--verbose] \
  [--diarization-model-path /path] [--diarization-model-repo org/repo] \
  [--diarization-num-speakers N]
```
Runs transcription and diarization in one pass. Word timestamps are enabled when --diarization is set so speaker labels align at the subsegment level. RTTM lines are printed to stdout.

WhisperAX shows full SpeakerKit integration:
- Pipeline version picker (Pyannote 4 or None)
- Speakers tab with per-segment speaker labels

```
SpeakerKit
├── SpeakerKit.swift             — public API (diarize, generateRTTM)
├── SpeakerKitModelManager.swift — download + load Pyannote models
├── PyannoteConfig.swift         — config + DiarizationOptionsProtocol
├── DiarizationResult.swift      — result type + transcription alignment
├── SpeakerSegment.swift
├── SpeakerInfo.swift
├── RTTMLine.swift
└── Pyannote/
    ├── PyannoteDiarizer.swift
    ├── SpeakerSegmenterModel.swift
    ├── SpeakerEmbedderModel.swift
    ├── SpeakerPreEmbedderModel.swift
    ├── SpeakerClustering.swift      — clustering protocol + config
    ├── VBxClustering.swift          — variational Bayes HMM
    ├── ClusteringAlgorithms.swift   — fast linkage
    └── MathOps.swift                — matrix ops (Accelerate)
ArgmaxCore
├── ModelDownloader.swift        — downloads + ModelInfo struct
├── ModelState.swift
└── (other shared utilities: Logging, ModelUtilities, etc.)
```

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Co-authored-by: ZachNagengast <znagengast@gmail.com>