Client: dual-channel label-merge (mic_file + system_file)

The backend shipped dual-channel mode; wire the client to it. We already capture
mic (you) and system (others) separately, so send them as two files instead of the
mono mix — fixing the misattribution at the source.

- SparkControlClient: labelMergeDual(mic_file, system_file, self_name, self_vad);
  multipart generalized to N files; shared POST/retry/decode extracted.
- SessionPackager.rebasedSelfVadData: chunk-local [{start,end}] for self_vad;
  sliceAudio reused for both tracks.
- TranscriptPipeline.process: dual-channel chunking (slice mic+system, rebase
  timeline + self_vad per chunk) when system audio is healthy; mono mixed-file
  fallback (self folded into the timeline) otherwise.
- VisualCapture.finish: write the full visual_timeline.json (remote + self merged)
  but return REMOTE (vision) segments only — self travels via the mic channel.
- TranscriptAssembler: rank mic_channel highest (the user's own track wins).
- VoiceprintStore: store the clean mic_channel self voiceprint.
- SessionController: pass mic/system URLs + remote timeline + channel self-spans +
  self_name + systemHealthy; self_vad.json now reflects the channel-verified spans.

Validated END-TO-END against the live backend on the real misattributing session:
'Go Bitcoin' (remote) is now attributed to Unknown_0, NOT the user; the user's own
lines come back source=mic_channel; per-channel ASR recovered fuller remote text.
36/36 XCTest (4 new: self_vad rebase, mic_channel ranking + voiceprint storage).

Ten31Transcripts/Session/SessionPackager.swift

@@ -46,6 +46,18 @@ enum SessionPackager {
         return try JSONSerialization.data(withJSONObject: flat, options: [])
     }
 
+    /// Clip self-VAD spans to `[start, end)` and rebase to chunk-local seconds, as
+    /// the `self_vad` array `[{start,end}]` for dual-channel `label-merge`.
+    static func rebasedSelfVadData(_ spans: [VADSpan], start: Double, end: Double) throws -> Data {
+        let flat: [[String: Any]] = spans.compactMap { span in
+            let s = max(span.start, start)
+            let e = min(span.end, end)
+            guard e > s else { return nil }
+            return ["start": s - start, "end": e - start]
+        }
+        return try JSONSerialization.data(withJSONObject: flat, options: [])
+    }
+
     /// Slice `[startSec, endSec)` of a 16 kHz mono WAV into `dest`.
     static func sliceAudio(from source: URL, startSec: Double, endSec: Double, to dest: URL) throws {
         let input = try AVAudioFile(forReading: source)