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).

Ten31TranscriptsTests/VoiceprintStoreTests.swift

@@ -23,6 +23,14 @@ final class VoiceprintStoreTests: XCTestCase {
         XCTAssertEqual(store.entries["Grant"]?.calls, 1)
     }
 
+    func testStoresMicChannelSelf() throws {
+        let url = tempURL(); defer { try? FileManager.default.removeItem(at: url) }
+        let store = VoiceprintStore(fileURL: url)
+        let json = #"{"duration":10,"speakers":[{"cluster":"mic","name":"Grant","source":"mic_channel","fingerprint":[0.5,0.6]}],"segments":[],"fingerprints":{"Grant":[0.5,0.6]},"models":{}}"#
+        store.update(with: try JSONDecoder().decode(LabelMergeResponse.self, from: Data(json.utf8)))
+        XCTAssertEqual(store.knownVoiceprints()["Grant"], [0.5, 0.6])   // clean self print stored
+    }
+
     func testPersistsAcrossInstancesAndIncrementsCalls() throws {
         let url = tempURL(); defer { try? FileManager.default.removeItem(at: url) }
         let store = VoiceprintStore(fileURL: url)