feat(plugins): request_on_demand() / end_on_demand() -- plugins ask for the screen in-process (#768)

* feat(plugins): request_on_demand() / end_on_demand() -- plugins ask for the screen in-process

Four plugins (birdnet-go, mqtt-notifications, on-air, pomodoro-timer) take
the screen by writing the display_on_demand_request mailbox, which the
display reads once a second while the control socket is up and which stage 5
removes. This is the in-process way in that stage needed.

- BasePlugin.request_on_demand(mode=None, duration=None, pinned=False) and
  end_on_demand(), safe from any thread, go through PluginManager to
  DisplayController.submit_plugin_on_demand, which only queues (at most 32)
  and wakes the render thread through ControlServer.wake(). The render
  thread applies them in _drain_control_commands, after socket commands,
  through _handle_on_demand_request, so they land within a frame; without a
  socket, on the next pending-changes pass.
- A plugin's stop ends only its own session; a mailbox stop still ends any.
- Both answer the request id, or None with no display in the process (web
  interface, check_plugin.py), a full queue, or a mock manager -- a plugin's
  cue to write the mailbox, which the display still reads.
- docs/PLUGIN_API_REFERENCE.md documents the hasattr pattern for plugins
  that must keep working on older cores; IPC_CONTROL_SOCKET.md and the
  CHANGELOG are updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(display): wire the on-demand handler only on a manager that has it

Tests and the golden traces stand in simpler plugin managers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/ipc/server.py

@@ -772,8 +772,22 @@ class ControlServer:
         """
         return self._pending.wait(timeout)
 
+    def wake(self) -> None:
+        """Wake the render thread as a queued command would, with nothing queued.
+
+        For work that reaches the display another way in the same process (a
+        plugin's on-demand request, ``DisplayController.submit_plugin_on_demand``):
+        the render thread returns from :meth:`wait_for_command` and drains,
+        and reads the caller's own queue there. Safe from any thread.
+        """
+        self._pending.set()
+
     def drain(self) -> List[QueuedCommand]:
-        """Every queued command, oldest first. Called from the render thread."""
+        """Every queued command, oldest first. Called from the render thread.
+
+        Clears the wake flag first, so anything queued (or woken for) while
+        this runs wakes the next wait again.
+        """
         commands: List[QueuedCommand] = []
         self._pending.clear()
         while True: