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/VisualCapture.swift

@@ -75,7 +75,10 @@ final class VisualCapture {
         }, to: durationSec)
 
         let artifact = (vision + selfSegs).sorted { $0.start < $1.start }
-        let names = Set(artifact.map { $0.name })
+        // Roster = everyone OCR'd (speaking or not) ∪ the names that produced segments,
+        // so the participant count reflects true call size even when few people were
+        // detected speaking. Drives "Auto" chunk sizing downstream.
+        let names = Set(artifact.map { $0.name }).union(observer.participantNames())
         let participants = names.sorted().map {
             VisualTimeline.Participant(name: $0, isSelf: $0 == selfName ? true : nil, aliases: nil)
         }