debounce terminal resize, add SIGWINCH, remove padding

Rapid resize during window drag sends many SIGWINCH signals causing
the shell to redraw repeatedly, corrupting xterm's render. Debounce
to 150ms so only the final size is sent.

NativePty.resize now explicitly sends SIGWINCH (signal 28) after
TIOCSWINSZ, matching the legacy Python implementation.

Removed 8px padding from ClidePtyView to eliminate potential
dimension mismatch between widget size and PTY cols/rows.

Default window size increased to 1600x900.

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

lib/src/pty/native_pty.dart

@@ -278,6 +278,10 @@ class NativePty {
       ..ref.wsCol = cols;
     _ioctl(_fd, _kTiocsWinsz, ws);
     calloc.free(ws);
+    // Explicitly signal the child to re-query its terminal size.
+    // macOS should auto-send SIGWINCH on TIOCSWINSZ, but the legacy
+    // Python implementation sent it explicitly for reliability.
+    _nativeKill(pid, 28); // SIGWINCH = 28 on macOS/Linux
   }
 
   /// Send a signal to the child.