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/TimelineBuilder.swift

@@ -15,9 +15,15 @@ final class TimelineBuilder {
     private let closeFrames: Int
     private var aliases: [String: String] = [:]      // normalized variant -> canonical
     private var states: [String: NameState] = [:]
+    private var observed: Set<String> = []           // every tile name seen (speaking or not)
     private var lastFrameT: Double = 0
     private(set) var segments: [VisualTimeline.Segment] = []
 
+    /// Every distinct participant name the adapter has OCR'd, whether or not they were
+    /// ever detected speaking — the call-size signal (drives "Auto" chunk sizing and a
+    /// complete participant roster, since speaking-detection is intentionally sparse).
+    var observedNames: [String] { observed.sorted() }
+
     init(openFrames: Int = 2, closeFrames: Int = 2) {
         self.openFrames = max(1, openFrames)
         self.closeFrames = max(1, closeFrames)
@@ -34,6 +40,9 @@ final class TimelineBuilder {
     func ingest(_ observations: [SpeakerObservation], at t: TimeInterval) {
         lastFrameT = t
 
+        // Record every tile seen (speaking or not) for the participant roster / call size.
+        for obs in observations where !obs.name.isEmpty { observed.insert(canonical(obs.name)) }
+
         // Best confidence per canonical name that is speaking this frame.
         var speaking: [String: Double] = [:]
         for obs in observations where obs.speaking && !obs.name.isEmpty {