fix: resolve CodeRabbit review and Codacy findings on #534

CodeRabbit raised six; all six were real.

The test double had drifted ahead of production. VisualTestDisplayManager
accepted set_scrolling_state(frame_hold=...) while DisplayManager did not,
so such a call passed every harness run and would raise TypeError on the
panel -- the one failure a safety harness exists to prevent. frame_hold
belongs to the change that adds it to DisplayManager (#523), so it moves
there and the double matches main again.

The harness swallowed exceptions from re-rendered frames. _settle_loop
re-renders a mode that came back blank, to give a scroll time to draw;
returning silently on a crash meant a mode that renders one good frame
and then explodes was reported as passing. Recorded on result.error now,
keeping the captured frame so the failure stays inspectable.

starlark-apps display() returned True after _display_frame() failed, so
the controller held a dead frame for the whole display_duration instead
of rotating on. _display_frame now returns bool on all three paths.

run_plugin_tests.py used env.setdefault for PYTHONPATH and
LEDMATRIX_CORE, so an inherited value won and the subprocess imported a
different core than the one under test -- ledmatrix-plugins#467 exactly.
Prepends PROJECT_ROOT and sets LEDMATRIX_CORE unconditionally.

The on-demand mailbox is polled after every frame, ~125x/second on a
scrolling mode, and the read is deliberately uncached, so it was that
many disk reads per second to find nothing. Floored at 250ms, which is
imperceptible for a web-UI click. Consuming it also deleted whatever was
present rather than what had just been processed, so a request posted
while the previous one was in flight was thrown away and never ran; the
delete is now keyed by request_id. That narrows the window rather than
closing it -- a true atomic claim needs a primitive the cache layer does
not offer, and the code says so rather than implying otherwise.

Codacy's 2 criticals were bandit B404/B603 on the subprocess call added
to run_plugin_tests.py. Fixed interpreter, argument list, no shell;
annotated with the repo's existing nosec convention. Bandit is clean on
the file.

Adds test/test_on_demand_mailbox.py (8), test_starlark_display_contract.py
(4) and two settle cases in test_harness_empty_claimed.py. 4, 4 and 2 of
those fail against the pre-fix code. Full suite: 3961 passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RRtqXDCnvnY6EQwhT5CV9

plugin-repos/starlark-apps/manager.py

@@ -714,9 +714,11 @@ class StarlarkAppsPlugin(BasePlugin):
                     self.logger.error(f"Failed to render app: {self.current_app.app_id}")
                     return False
 
-            # Display current frame
-            self._display_frame()
-            return True
+            # Display current frame. The result is propagated: a failed frame
+            # update is not a displayed frame, and returning True regardless
+            # told the controller the mode had rendered, so it held the dead
+            # frame for the whole display_duration instead of rotating on.
+            return self._display_frame()
 
         except Exception as e:
             self.logger.error(f"Error displaying Starlark app: {e}")
@@ -843,10 +845,13 @@ class StarlarkAppsPlugin(BasePlugin):
             self.logger.error(f"Error loading frames for {app.app_id}: {e}")
             return False
 
-    def _display_frame(self) -> None:
-        """Display the current frame of the current app."""
+    def _display_frame(self) -> bool:
+        """Display the current frame of the current app.
+
+        :returns: whether a frame actually reached the display manager.
+        """
         if not self.current_app or not self.current_app.frames:
-            return
+            return False
 
         try:
             current_time = time.time()
@@ -864,8 +869,11 @@ class StarlarkAppsPlugin(BasePlugin):
                 )
                 self.current_app.last_frame_time = current_time
 
+            return True
+
         except Exception as e:
             self.logger.error(f"Error displaying frame: {e}")
+            return False
 
     def install_app(self, app_id: str, star_file_path: str, metadata: Optional[Dict[str, Any]] = None, assets_dir: Optional[str] = None) -> bool:
         """