Chunk overlap + overlap-aware stitching

Chunks were contiguous (start = prev end) with a naïve offset-concat stitch — no
overlap. That cut sentences at boundaries, denied the diarizer context at edges, and
let one voice split across chunks (the MH/Unknown_0 problem). Now each ~150s body is
sliced with a 15s margin on both sides ([bodyStart-15, bodyEnd+15]); the stitcher
keeps a segment only in the chunk that owns its MIDPOINT (body region) and drops it
from the neighbour's margin — so boundary-spanning speech is seen whole by the
backend and kept exactly once.

- SessionPackager.PlannedChunk gains bodyStart/bodyEnd; planChunks adds overlapSeconds.
- TranscriptAssembler.ChunkResult carries body bounds (defaults keep-all → no-overlap
  behaviour preserved for existing callers); assemble dedups by midpoint-in-body.
- TranscriptPipeline passes body bounds through.

Complements (doesn't replace) the fragment-smoothing + reconciliation safety nets;
this is the upstream fix. ~+20% backend audio per interior chunk. 63/63 XCTest
(new: overlap window layout + boundary-segment dedup).

Ten31TranscriptsTests/Phase5Tests.swift

@@ -8,11 +8,33 @@ final class Phase5Tests: XCTestCase {
         XCTAssertEqual(c[0].end, 70, accuracy: 0.001)
     }
 
-    func testPlanChunksLong() {
-        let c = SessionPackager.planChunks(durationSec: 400, chunkSeconds: 150)
+    func testPlanChunksLongOverlapsWindowsWithContiguousBodies() {
+        let c = SessionPackager.planChunks(durationSec: 400, chunkSeconds: 150, overlapSeconds: 15)
         XCTAssertEqual(c.count, 3)
-        XCTAssertEqual(c[0].start, 0); XCTAssertEqual(c[0].end, 150)
-        XCTAssertEqual(c[1].start, 150); XCTAssertEqual(c[2].end, 400)
+        // Owned bodies tile the call with no gaps/overlap.
+        XCTAssertEqual(c[0].bodyStart, 0);   XCTAssertEqual(c[0].bodyEnd, 150)
+        XCTAssertEqual(c[1].bodyStart, 150); XCTAssertEqual(c[1].bodyEnd, 300)
+        XCTAssertEqual(c[2].bodyEnd, 400)
+        // Sliced windows overlap by the margin (and clamp at the ends).
+        XCTAssertEqual(c[0].start, 0); XCTAssertEqual(c[0].end, 165)   // +15 trailing
+        XCTAssertEqual(c[1].start, 135)                                // -15 leading
+        XCTAssertLessThan(c[1].start, c[0].end)                        // windows overlap
+        XCTAssertEqual(c[2].end, 400)                                  // clamped
+    }
+
+    func testAssembleDedupsOverlapByBody() {
+        // A segment at global 152–156 sits in chunk1's body but also in chunk0's
+        // trailing margin (overlap). It must be kept exactly once (by chunk1).
+        let r0 = #"{"duration":165,"speakers":[{"cluster":"S0","name":"A","source":"visual","overlap_confidence":0.9}],"segments":[{"start_ms":152000,"end_ms":156000,"speaker":"A","text":"boundary"}],"fingerprints":{},"models":{}}"#
+        let r1 = #"{"duration":180,"speakers":[{"cluster":"S0","name":"A","source":"visual","overlap_confidence":0.9}],"segments":[{"start_ms":17000,"end_ms":21000,"speaker":"A","text":"boundary"}],"fingerprints":{},"models":{}}"#
+        let c0 = try! JSONDecoder().decode(LabelMergeResponse.self, from: Data(r0.utf8))
+        let c1 = try! JSONDecoder().decode(LabelMergeResponse.self, from: Data(r1.utf8))
+        let asm = TranscriptAssembler.assemble(sessionId: "s", app: "meet", chunks: [
+            .init(chunkStart: 0, response: c0, bodyStart: 0, bodyEnd: 150),
+            .init(chunkStart: 135, response: c1, bodyStart: 150, bodyEnd: 300),
+        ])
+        XCTAssertEqual(asm.speakersFile.segments.count, 1)                       // deduped
+        XCTAssertEqual(asm.speakersFile.segments[0].start, 152, accuracy: 0.01)
     }
 
     func testRebaseClipsAndRebases() throws {