[BUG] App-level is_mounted guards in the TUI are dead code (always truthy)

[MBemera]

Describe the bug

App.is_mounted is a method taking a widget, not the Widget.is_mounted
property:

# textual/app.py
def is_mounted(self, widget: Widget) -> bool:
    return widget in self._registry

StrixTUIApp subclasses App, so a bare self.is_mounted is a bound method
object, always truthy, and not self.is_mounted is always False. All 13 guards
using it never fire: lines 870, 942, 957, 989, 1113, 1470, 1545, 1682, 1702,
1751, 1769, 1785, 1944. (widget.is_mounted at 1931 is on a Widget and is
correct.)

Worst case is app.py:950 — the dead guard lets query_one run against a missing
screen, raise, and get caught by an except that re-schedules via
call_after_refresh, looping instead of returning.

To Reproduce

1. uv run mypy strix/ → Function "is_mounted" could always be true in boolean context, plus Statement is unreachable after 7.
2. uv run pyright strix/ → 13 × reportUnnecessaryComparison.

Expected behavior

if not <guard>: return should return when the app isn't up. App.is_running
is the property these guards want.

System Information:

• OS: environment-independent (static logic bug)
• Strix Version or Commit: 1.3.1 / 27f9750
• Python Version: 3.12+
• LLM Used: n/a
• textual: 6.2.1