fix(diarizer/offline): propagate cancellation to workers

[CiprianSpiridon]

Why is this change needed?

OfflineDiarizerManager.prepare launches segmentation and embedding inference as
detached tasks. Detached tasks do not inherit cancellation from the caller, so
cancelling process(URL) during a long recording can leave Core ML work running
after the caller has given up. That prevents clients from reliably preempting
offline diarization when a higher-priority audio or transcription task needs the
device.

Changes

• Run segmentation and embedding extraction as children of one throwing task
  group rather than as detached tasks.
• Cancel the sibling worker and terminate the segmentation stream when either
  worker fails or is cancelled.
• Keep the structured scope alive until both inference workers have terminated,
  so returning CancellationError is also the shutdown boundary.
• Add a real-file regression that cancels after inference begins, requires the
  call to return within two seconds, and verifies that progress does not continue
  afterward.

There is no public API change. Successful diarization still returns the same
prepared segmentation and embedding results.

Test Plan

• swift format lint --configuration .swift-format Sources/FluidAudio/Diarizer/Offline/Core/OfflineDiarizerManager.swift Tests/FluidAudioTests/Diarizer/Offline/OfflineDiarizerCancellationTests.swift
• swift test --parallel --num-workers 8 (2,332 discovered tests, exit 0)
• FLUIDAUDIO_CANCELLATION_TEST_AUDIO=/private/tmp/EN2002a.Mix-Headset.wav swift test --filter OfflineDiarizerCancellationTests
  • Passed with the project's official AMI EN2002a single-distant-microphone
    meeting and locally cached production diarization models.
  • Cancellation completed in 0.918 seconds on an Apple M3 Max running macOS
    26.3, with no progress after the call returned.

The real-file regression skips when FLUIDAUDIO_CANCELLATION_TEST_AUDIO or the
offline diarization models are unavailable, so the repository does not need to
bundle a large meeting recording or model artifacts.

Notes

The repository-wide format and test commands still emit existing warnings from
untouched files, including the unhandled sample_medical.arpa test fixture. The
changed files are format-clean, and the complete test command exits successfully.