Collapse adjacent same-speaker segments after reconciliation

Fragments reabsorbed by smoothFragments (e.g. "I" then "need to switch it
back") were left as separate transcript lines. Add SpeakerReconciler.mergeAdjacent
to join consecutive same-speaker segments within 2s, concatenating their text.

Wire it into SessionController.finishBackend AFTER reconcile/LLM naming. The
collapse needs no LLM, so finishBackend no longer early-returns when the gateway
has no chat model — it runs the collapse and re-persists speakers.json
unconditionally, gating only the reconcile and recap passes on the model.

Ten31Transcripts/Session/SpeakerReconciler.swift

@@ -82,6 +82,28 @@ enum SpeakerReconciler {
                             speakers: speakers, segments: result, models: file.models)
     }
 
+    /// Collapse consecutive segments from the SAME speaker separated by ≤ `maxGap`
+    /// seconds into one, joining their text — so fragments reabsorbed by smoothing
+    /// (e.g. "I" then "need to switch it back") read as a single clean line. Pure.
+    static func mergeAdjacent(_ file: SpeakersFile, maxGap: Double = 2.0) -> SpeakersFile {
+        let sorted = file.segments.sorted { $0.start < $1.start }
+        guard !sorted.isEmpty else { return file }
+        var out: [SpeakersFile.Segment] = []
+        for s in sorted {
+            if var last = out.last, last.speaker == s.speaker, s.start - last.end <= maxGap {
+                let joined = [last.text, s.text].compactMap { $0?.trimmingCharacters(in: .whitespaces) }
+                    .filter { !$0.isEmpty }.joined(separator: " ")
+                last = .init(start: last.start, end: max(last.end, s.end), speaker: s.speaker,
+                             text: joined.isEmpty ? nil : joined)
+                out[out.count - 1] = last
+            } else {
+                out.append(s)
+            }
+        }
+        return SpeakersFile(sessionId: file.sessionId, app: file.app, durationSec: file.durationSec,
+                            speakers: file.speakers, segments: out, models: file.models)
+    }
+
     // MARK: - Voiceprint merge (pure)
 
     static func protectedNames(_ file: SpeakersFile, selfName: String) -> Set<String> {