fix: resolve terminal lifecycle race conditions

1. Lock pyte screen initialization in open() to prevent races with
   concurrent _update_screen() calls

2. Reorder session registration: call open() BEFORE adding to
   sessions/routes dicts, so sessions are fully initialized before
   other code can access them

3. Add clarifying comment that PTY resize completes before pyte resize

These fixes prevent dimension mismatches between PTY and pyte screen
that could cause content wrapping in screenshots.

src/textual_webterm/terminal_session.py

@@ -64,9 +64,10 @@ class TerminalSession(Session):
 
     async def open(self, width: int = 80, height: int = 24) -> None:
         log.info("Opening terminal session %s with command: %s", self.session_id, self.command)
-        # Initialize pyte screen with the requested size
-        self._screen = pyte.Screen(width, height)
-        self._stream = pyte.Stream(self._screen)
+        # Initialize pyte screen with the requested size (under lock to prevent races)
+        async with self._screen_lock:
+            self._screen = pyte.Screen(width, height)
+            self._stream = pyte.Stream(self._screen)
 
         pid, master_fd = pty.fork()
         self.pid = pid
@@ -99,9 +100,10 @@ class TerminalSession(Session):
         fcntl.ioctl(self.master_fd, termios.TIOCSWINSZ, buf)
 
     async def set_terminal_size(self, width: int, height: int) -> None:
+        # First resize the PTY (blocking call in executor)
         loop = asyncio.get_running_loop()
         await loop.run_in_executor(None, self._set_terminal_size, width, height)
-        # Resize pyte screen to match
+        # Then resize pyte screen to match (after PTY resize completes)
         async with self._screen_lock:
             self._screen.resize(height, width)