fix(core): unique snapshot temp name, honour on-demand requests, skip empty starlark

The preview snapshot wrote through a fixed "<snapshot>.tmp". /tmp is
world-writable and sticky, and the display service runs as a different user
from the tooling, so a leftover temp owned by anyone else became unopenable
even by root -- fs.protected_regular refuses O_CREAT on a foreign file in a
sticky directory. The preview and the health check's liveness proxy then froze
until someone deleted the file by hand; on the test rig that meant 23 hours of
a healthy display reporting "hardware: stale". Now uses tempfile.mkstemp with
cleanup on failure, matching the hardware-status write a few hundred lines
above. Closes #528.

_poll_on_demand_requests read its mailbox with max_age=3600, and get() defaults
the in-memory TTL to max_age -- so the first request was pinned in memory for an
hour and every later poll returned that stale copy. No second on-demand request
was honoured until the service restarted, while the API kept returning 200.
get() already documents memory_ttl=0 for exactly this cross-process case.
The consumed request is also now deleted: leaving it on disk meant a restart
replayed the previous request, activated it, and ignored the one the caller had
just made. Closes #530.

starlark-apps returned None from display() when it has no app to show, which is
the state of every install without Pixlet and of a fresh one before any app is
added. The controller only skips on a boolean False, so that held a black panel
for the full display_duration instead of rotating on. Closes #456 (core side).

Verified on the rig: two consecutive on-demand requests with no restart between
them are both activated, where the second was previously dropped in silence.

plugin-repos/starlark-apps/manager.py

@@ -681,12 +681,18 @@ class StarlarkAppsPlugin(BasePlugin):
                 if app.is_enabled() and app.should_render(current_time):
                     self._render_app(app, force=False)
 
-    def display(self, force_clear: bool = False) -> None:
+    def display(self, force_clear: bool = False) -> bool:
         """
         Display current Starlark app.
 
         This method is called during the display rotation.
         Displays frames from the currently active app.
+
+        Returns False when there is no app to show -- which is the state of
+        every install without Pixlet, and of a fresh one before any app is
+        added. The display controller only skips a mode on a boolean False
+        (it checks isinstance(result, bool)), so returning None held a black
+        panel for the full display_duration instead of rotating on.
         """
         try:
             if force_clear:
@@ -699,20 +705,22 @@ class StarlarkAppsPlugin(BasePlugin):
             if not self.current_app:
                 # No apps available
                 self.logger.debug("No Starlark apps to display")
-                return
+                return False
 
             # Render app if needed
             if not self.current_app.frames:
                 success = self._render_app(self.current_app, force=True)
                 if not success:
                     self.logger.error(f"Failed to render app: {self.current_app.app_id}")
-                    return
+                    return False
 
             # Display current frame
             self._display_frame()
+            return True
 
         except Exception as e:
             self.logger.error(f"Error displaying Starlark app: {e}")
+            return False
 
     def _select_next_app(self) -> None:
         """Select the next enabled app for display."""