feat(app): expose experimental diarization tuning in Settings (#165 phase 5)

[pasrom]

Summary

• Adds an "Experimental: Diarization Tuning" DisclosureGroup in Settings → Speakers, exposing 5 OfflineDiarizerConfig knobs (clusterThreshold, warmStartFa, warmStartFb, minSegmentDurationSeconds, excludeOverlap).
• Red warning banner + Reset to defaults button to mitigate footgun: warm-start values are research-tuned and changing them blindly degrades quality. A small orange dot next to the disclosure label keeps a non-default state visible even when the group is collapsed.
• New OfflineDiarizerTuning struct decouples FluidOfflineProcessor from AppSettings/UserDefaults so the plumbing is unit-testable.

Implements phase 5 of #165 (Variant C: power-user, with safety rails).

Choices worth flagging

• The disclosure renders only when diarizerMode == .offline. The 5 knobs are all on the offline pipeline; surfacing them while Sortformer is active would be misleading since they have no effect.
• Reset button uses settings.diarizerTuningIsAllDefaults (a computed Bool) for its disabled state. The same flag drives the orange "non-default" dot.

Test plan

• AppSettings round-trip + reset tests (new testDiarizerTuningDefaults, testDiarizerTuningRoundTrip, testResetDiarizerTuningRestoresDefaults).
• OfflineDiarizerTuning.defaults matches FluidAudio's Clustering.community / Embedding.community exactly (testTuningStructDefaults).
• OfflineDiarizerTuning.apply(to:) and FluidOfflineProcessor.makeConfig produce expected configs without disturbing other knobs.
• DisclosureGroup collapsed by default; Reset button disabled when at defaults / enabled when changed; section hidden for Sortformer.
• swift test green outside the pre-existing flaky MenuBarIconSnapshotTests (also failing on stashed main).
• ./scripts/lint.sh clean.

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