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.

Ten31TranscriptsTests/Phase5Tests.swift

@@ -37,6 +37,33 @@ final class Phase5Tests: XCTestCase {
         XCTAssertEqual(asm.speakersFile.segments[0].start, 152, accuracy: 0.01)
     }
 
+    func testChunkModeResolvesBodyLength() {
+        // Fixed presets ignore participant count.
+        XCTAssertEqual(ChunkMode.standard.bodySeconds(participantCount: 99), 150)
+        XCTAssertEqual(ChunkMode.largeGroup.bodySeconds(participantCount: 2), 60)
+        XCTAssertEqual(ChunkMode.fine.bodySeconds(participantCount: nil), 90)
+        // Auto: >4 detected → 60s, ≤4 → 150s, unknown → 150s.
+        XCTAssertEqual(ChunkMode.auto.bodySeconds(participantCount: 6), 60)
+        XCTAssertEqual(ChunkMode.auto.bodySeconds(participantCount: 4), 150)
+        XCTAssertEqual(ChunkMode.auto.bodySeconds(participantCount: nil), 150)
+    }
+
+    func testChunkOverlapScalesWithBody() {
+        XCTAssertEqual(ChunkMode.overlapSeconds(forBody: 150), 15)   // capped
+        XCTAssertEqual(ChunkMode.overlapSeconds(forBody: 60), 8)     // floored (60*0.12=7.2→8)
+        XCTAssertEqual(ChunkMode.overlapSeconds(forBody: 90), 11)    // 90*0.12=10.8→11
+    }
+
+    func testPlanChunksShortBodyChunksAShortCall() {
+        // A 100s call would be ONE chunk at the 2.5-min default, but at a 60s body it
+        // splits — so "Large group" actually re-chunks medium calls.
+        let c = SessionPackager.planChunks(durationSec: 100, chunkSeconds: 60,
+                                           overlapSeconds: 8, thresholdSec: 72)
+        XCTAssertEqual(c.count, 2)
+        XCTAssertEqual(c[0].bodyStart, 0); XCTAssertEqual(c[0].bodyEnd, 60)
+        XCTAssertEqual(c[1].bodyStart, 60); XCTAssertEqual(c[1].bodyEnd, 100)
+    }
+
     func testDropStuckSpansRemovesWholeCallCue() {
         let segs = [
             VisualTimeline.Segment(start: 0, end: 1900, name: "Grant Gilliam", confidence: 1, source: "vision"), // stuck whole-call tile