Make diarization chunk length configurable (Auto + presets)

Chunk size was hardcoded at 2.5-min bodies. Add a Settings control:
Auto / Standard 2.5min / Large group 60s / Fine 90s. Shorter chunks keep fewer
simultaneous speakers per window (Sortformer resolves ~4/chunk), useful for large
calls, at some cost to speed and cross-chunk voice matching.

- ChunkMode (new, pure/testable): mode → body seconds; Auto picks 60s when >4
  participants were detected, else 150s; overlap + single-chunk threshold scale
  with the body length.
- AppSettings.chunkMode (+ typed `chunk`); SettingsView picker with explanation.
- TranscriptPipeline.process gains chunkSeconds; derives overlap/threshold from it.
- SessionController resolves the body from the setting + the session's detected
  participant count (visual_timeline participants) for both send + re-process.
- Participant roster now counts EVERY tile OCR'd, not just who spoke
  (TimelineBuilder.observedNames → VisualObserver → VisualCapture), so the Auto
  call-size signal is meaningful even though speaking-detection is sparse.

Tests: ChunkMode resolution, overlap scaling, short-body re-chunking. 69 pass.

Ten31Transcripts/Visual/VisualObserver.swift

@@ -114,6 +114,10 @@ final class VisualObserver: NSObject, SCStreamDelegate, SCStreamOutput {
         queue.sync { builder.mergeSelfSpans(spans, selfName: selfName) }
     }
 
+    /// Every distinct participant name OCR'd over the session (read on the builder's
+    /// queue; safe to call after `stop`).
+    func participantNames() -> [String] { queue.sync { builder.observedNames } }
+
     // MARK: - SCStreamOutput (on `queue`)
 
     func stream(_ stream: SCStream, didOutputSampleBuffer sampleBuffer: CMSampleBuffer,