refactor(terminal): seams for a second terminal pane (input socket map, targeted links, copy, paste)

No behavior change. Prepares the split pane's second terminal (and later
grid tiles) to share what today only the primary terminal has:

- _inputSocketFor/_registerInputSocket/_unregisterInputSocket: the
  exactly-once input queue, its ACK handling and the redelivery sweep now
  deliver over any registered socket bound to a session, not only
  this._ws. ACKs are routed by the receiving socket's session; silence is
  judged per socket; a stale handle cannot unregister its replacement.
- registerFilePathLinkProvider, cleanedTerminalSelection,
  copyTerminalSelection and _handleImagePaste take an optional target
  terminal and session (defaults: the primary pane).
- _focusedPane() is the one place to ask which pane the keyboard is in
  (primary only, for now); _forEachTile() replaces the _splitPane special
  cases in the font, family, weight, skin and resize paths.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/input-size-limit.test.ts

@@ -97,7 +97,7 @@ describe('the client never queues or keeps an undeliverable frame', () => {
   });
 
   it('drops a frame the WebSocket refused with an error ACK', () => {
-    const handler = appSource.slice(appSource.indexOf('_onWsInputAck(seq, msg) {'));
+    const handler = appSource.slice(appSource.indexOf('_onWsInputAck(seq, msg, sessionId = this._wsSessionId) {'));
     expect(handler.slice(0, 600)).toMatch(/if \(msg && msg\.err\)/);
   });