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/AdapterRegistry.swift

@@ -0,0 +1,17 @@
+import Foundation
+
+/// Maps a detected call app to its screen-reading adapter. One place to wire new
+/// adapters; `VisualObserver` is constructed from whatever this returns when the
+/// live visual capture is integrated into the session.
+enum AdapterRegistry {
+    /// The adapter for a detected app, or nil if that app has no visual adapter
+    /// yet (the session then runs audio-only — graceful degradation).
+    static func adapter(for app: CallDetector.DetectedApp) -> (any AppAdapter)? {
+        switch app {
+        case .signal: return SignalAdapter()
+        case .meet:   return MeetAdapter()
+        case .zoom:   return ZoomAdapter()
+        case .teams:  return TeamsAdapter()
+        }
+    }
+}