Force redraw on reconnect and speed up screenshots

- Send Ctrl+L and resize on reconnect to avoid black screens
- Increase replay buffer to 256KB
- Add get_screen_has_changes for non-destructive dirty checks
- Tighten screenshot cache TTLs and SSE debounce
- Update tests for new behavior and timings

src/textual_webterm/terminal_session.py

@@ -25,7 +25,7 @@ if TYPE_CHECKING:
 log = logging.getLogger("textual-web")
 
 # Maximum bytes to keep in replay buffer for reconnection
-REPLAY_BUFFER_SIZE = 64 * 1024  # 64KB
+REPLAY_BUFFER_SIZE = 256 * 1024  # 256KB
 
 # Default screen size for pyte emulator
 DEFAULT_SCREEN_WIDTH = 132