feat(split-pane): add SplitTerminalPane class for Pane B

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

config/test-suites.ts

@@ -30,6 +30,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/capture-load-window.browser.test.ts',
   'test/capture-geometry-retry.browser.test.ts',
   'test/codex-predictive-echo.test.ts', // also needs a real codex binary
+  'test/split-pane-terminal.browser.test.ts',
 ];
 
 /**