Adapters: add Meet, Zoom, Teams (coloured border) + adapter registry

Front-loads the remaining visual adapters per the Signal→Meet→Zoom priority.
All three reuse GridCallAnalyzer's coloured-border (saturated) detection path
and share the new bottom-left name anchor:

- GridCallAnalyzer: generalise nameAtBottom:Bool into a NameAnchor enum
  (.bottomCenter for Signal's centered footer, .bottomLeft for Meet/Zoom/Teams
  where the name hugs the tile's bottom-left corner, .center for completeness).
  tileRect estimates the tile up-and-right of a bottom-left name.
- MeetAdapter (Google-blue ring, browser-hosted), ZoomAdapter (green/yellow
  border), TeamsAdapter (violet ring): coloured-border on, white-border off,
  bottom-left names. Geometry constants are first-pass pending real fixtures.
- AdapterRegistry.adapter(for:) maps CallDetector.DetectedApp → AppAdapter so
  VisualObserver can be constructed when live visual capture is wired in;
  unmapped apps degrade to audio-only.

Synthetic 4-tile tests: Meet picks each blue-bordered speaker with no
adjacent-tile bleed, Zoom picks the green-bordered speaker, and Signal's
white-only detector correctly ignores a coloured border. 18/18 XCTest pass.

Ten31Transcripts/Adapters/SignalAdapter.swift

@@ -19,7 +19,7 @@ struct SignalAdapter: AppAdapter {
         // 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.nameAnchor = .bottomCenter
         config.detectWhiteBorder = true
         config.detectColoredBorder = false
         config.tileExpandX = 2.4