niri-scratchpad: stop screencasts on stash; harden acceptance spawn wait

Stashing a window now stops an active screencast of it, rather than leaving the
consumer on a frozen last frame (a stashed window is never rendered). Covers
both stash paths - Mod+M and Mod+Shift+M's hide branch - by diffing which
windows left the layout across the action, so scratchpad_show's branch logic
isn't duplicated in the caller. Dynamic-target casts are retargeted to Nothing
rather than ended; that carve-out is documented inline.

Also widen the acceptance harness spawn wait to 30s: each test launches its own
nested niri, and a cold alacritty start occasionally exceeded the old 10s.
Deliberately not a retry, which could double-spawn and corrupt window counts.

117 unit tests and 8/8 live acceptance pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/niri-scratchpad/tests/acceptance.py

@@ -97,17 +97,26 @@ class Niri:
             time.sleep(0.15)
         return self.windows()
 
-    def spawn_window(self):
-        """Spawn an alacritty window and return its id once it appears."""
+    def spawn_window(self, timeout=30):
+        """Spawn an alacritty window and return its id once it appears.
+
+        The wait is deliberately generous: every test launches its own nested
+        niri, so a cold alacritty start can occasionally be slow under that
+        load. Do NOT convert this into a retry — re-issuing the spawn risks a
+        second window arriving late and corrupting the window counts the tests
+        assert on. A slow test beats a flaky one.
+        """
         before = {w["id"] for w in self.windows()}
         self.action("spawn", "--", "alacritty")
         t0 = time.time()
-        while time.time() - t0 < 10:
+        while time.time() - t0 < timeout:
             new = [w for w in self.windows() if w["id"] not in before]
             if new:
                 return new[0]["id"]
             time.sleep(0.15)
-        raise RuntimeError("spawned window never appeared")
+        raise RuntimeError(
+            f"spawned window never appeared within {timeout}s "
+            f"(compositor alive={self.alive()})")
 
     def by_id(self, i):
         return next((w for w in self.windows() if w["id"] == i), None)