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/Backend/VoiceprintStore.swift

@@ -48,9 +48,10 @@ final class VoiceprintStore {
             guard !Self.isUnknown(sp.name) else { continue }
             let acceptable: Bool
             switch sp.source {
-            case "visual":     acceptable = (sp.overlapConfidence ?? 0) >= minOverlapToStore
-            case "voiceprint": acceptable = true            // already matched a known print
-            default:           acceptable = false           // unmatched
+            case "mic_channel": acceptable = true           // the user's own clean mic voiceprint
+            case "visual":      acceptable = (sp.overlapConfidence ?? 0) >= minOverlapToStore
+            case "voiceprint":  acceptable = true            // already matched a known print
+            default:            acceptable = false           // unmatched
             }
             guard acceptable, let vector = sp.fingerprint ?? response.fingerprints[sp.name],
                   !vector.isEmpty else { continue }