defer PTY spawn until first TerminalView resize

The PTY was spawning at 80x24 defaults before TerminalView had laid
out, then the real resize caused visual artifacts (ghost lines, broken
reflow). Now spawn triggers on the first onResize callback when real
dimensions are known — matching the pty-spike's proven pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-04-30T20:42:24Z",
+  "exported_at": "2026-05-01T07:24:10Z",
   "decisions": [
     {
       "id": "D-1",

lib/builtin/claude/src/claude_pane.dart

@@ -52,12 +52,15 @@ class _ClaudePaneState extends State<ClaudePane> {
   String _statusLine = 'attaching…';
 
   @override
+  bool _spawned = false;
+
   void initState() {
     super.initState();
     _terminal = Terminal(maxLines: _maxLines);
     _terminal.onOutput = _onOutput;
     _terminal.onResize = _onResize;
-    WidgetsBinding.instance.addPostFrameCallback((_) => _spawnWhenReady());
+    // Don't spawn here — wait for the first onResize from TerminalView
+    // so the PTY gets real dimensions, not 80x24 defaults.
   }
 
   @override
@@ -201,6 +204,12 @@ class _ClaudePaneState extends State<ClaudePane> {
   }
 
   void _onResize(int cols, int rows, int _, int __) {
+    if (!_spawned) {
+      // First resize — TerminalView has real dimensions now.
+      _spawned = true;
+      _spawnWhenReady();
+      return;
+    }
     final id = _paneId;
     if (id == null) return;
     _ipc()?.request('pane.resize', args: {'id': id, 'cols': cols, 'rows': rows});