feat(ipc)!: remove the cache-key mailboxes (control socket stage 5)

The control socket is now the only way the web interface sends the display a
command. The display stops reading display_on_demand_request and
plugin_error_clear_request, and the web interface stops writing them.

- Display: no mailbox poll (MailboxWatch, the 1 s / 0.25 s cadence,
  _consume_on_demand_request, the deprecation log) and no persisted
  display_on_demand_processed_id guard; the error publisher reads no clear
  request. CacheManager.file_signature and MailboxWatch are removed.
- A write to either retired key is dropped by CacheManager.save_cache and
  logged once per writer, naming the plugin from the call stack (or the
  request's plugin_id), with the API to move to.
- Web: on-demand start with no display listening starts the service (when
  start_service) and sends the request again once the socket answers (45 s,
  10 s for a running service without a socket yet); every other failure is
  a 503 (400 for invalid_args). Stop answers 503 when no display listens,
  unless stop_service. errors/clear answers 503 with a reason-specific
  message instead of writing a request; clear_pending is always false.
  src.ipc.client.should_fall_back is replaced by display_not_listening.
- Kept: display_current_state, display_on_demand_state,
  plugin_runtime_snapshot and the heartbeat (read whenever the socket cannot
  answer), and display_on_demand_config (the display's resume record).

Tests: mailbox-only tests removed (test_on_demand_mailbox.py, the mailbox
cadence, file_signature and MailboxWatch tests); tests that injected
requests through the mailbox now use the socket queue or a plugin's
in-process request. The run-loop harness sends on-demand requests over its
fake control socket, so four golden traces change: on-demand starts and
stops land at the request instant instead of the next 0.25 s mailbox look
(one frame fewer on the screen they end), and in vegas.json within one
frame instead of 263 ms, which shifts the later 1 s-throttled WiFi-notice
check by under a second.

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

test/_run_loop_harness.py

@@ -155,13 +155,6 @@ class FakeCache:
         self._writes += 1
         self._written[key] = self._writes
 
-    def file_signature(self, key):
-        """CacheManager.file_signature: None without a file, else a value
-        that changes with every write."""
-        if key not in self.data:
-            return None
-        return (self._written.get(key, 0), 0, 0)
-
     def delete(self, key):
         self.data.pop(key, None)
 
@@ -729,10 +722,23 @@ class RunLoopHarness:
         self.controller.available_modes.append(mode)
 
     def on_demand_request(self, t: float, request_id: str, action: str = "start", **fields):
+        """An on-demand start or stop from the web interface at ``t``: a
+        command on the control socket (served for the run if no test did),
+        which is the only way the web interface reaches the display."""
+        from src.ipc.contract import Command, parse_args
+        from src.ipc.server import QueuedCommand
+
+        server = self.controller._control_server
+        if not isinstance(server, FakeControlServer):
+            server = self.control_socket()
+        cmd = Command.ON_DEMAND_START if action == "start" else Command.ON_DEMAND_STOP
+        command = QueuedCommand(request_id=request_id, cmd=cmd,
+                                args=parse_args(cmd, fields if action == "start" else {}),
+                                received_at=0.0)
+
         def post():
             self.log("request", f"{action}:{request_id}")
-            self.cache.set("display_on_demand_request",
-                           {"request_id": request_id, "action": action, **fields})
+            server.queue.append(command)
         self.clock.at(t, post)
 
     def restore_on_demand(self, plugin_id: str, mode: Optional[str] = None,