Signal: detect the white speaking border (not a coloured one)

Signal's active-speaker cue is a 3px #ffffff rounded border (saturation ≈ 0),
which the saturation-based highlight detector could never see. Per the
Signal-Desktop source review:

- FrameSampler.thinWhitePoints: grid-sample near-white pixels that sit on a
  THIN structure (a non-white pixel within edgeGap on some axis) so a border/
  ring counts but a solid white blob (face, bright video) does not.
- GridCallAnalyzer: combine coloured (saturated) + white (thin) highlight
  pixels; exclude name-text regions so the white footer name can't be mistaken
  for the border; estimate the tile UP from the name footer (nameAtBottom);
  attribute each highlight pixel to exactly one tile by containment (nearest
  centre as tiebreak) so a border can't bleed into an adjacent tile.
- SignalAdapter: white border on, coloured off, name-at-bottom geometry.

Synthetic 4-tile harness now isolates each speaker with no adjacent-tile bleed;
all 15 XCTest cases pass. Real-screenshot geometry calibration still pending.

Ten31Transcripts/Adapters/SignalAdapter.swift

@@ -14,9 +14,16 @@ struct SignalAdapter: AppAdapter {
 
     init() {
         var config = GridCallAnalyzer.Config()
-        // Signal tiles are squarish with initials centred; tune with fixtures.
-        config.tileExpandX = 1.6
-        config.tileExpandY = 1.8
+        // Signal's speaking cue is a 3px WHITE rounded border (not coloured); the
+        // name is a bottom footer, so the tile extends up from it. Geometry tuned
+        // with real fixtures. (Gotchas, per Signal source: NO border in 1:1 calls —
+        // fall back to mic-VAD/audio pill — and in Speaker view the large tile is
+        // the speaker; both handled at a higher level later.)
+        config.nameAtBottom = true
+        config.detectWhiteBorder = true
+        config.detectColoredBorder = false
+        config.tileExpandX = 2.4
+        config.tileExpandY = 4.8
         self.analyzer = GridCallAnalyzer(config: config)
     }