display-demo: stop drawing a cursor and stop blocking on the mouse

Two real bugs visible in a normal `zig build run-x86-64` boot (but not in the
display-demo test, which spawns no input service):

  - Two cursors. The demo drew its own cursor layer while the display service
    now draws one too (its mouse-listener thread). The demo's went through the
    client IPC protocol and lagged; the service's is in-process and tracks
    tightly — "one responds better than the other."

  - Animation frozen until the mouse moves. The demo called a *blocking*
    `mouse.next()` (ipc_reply_wait) inside its animation loop, so the sliding
    box advanced only one frame per mouse event. In the display-demo test
    there is no input service, so subscribeMouse() returned null and the loop
    ran free on its 30ms timer — which is exactly why the test passed while
    the real boot was broken.

The compositor now owns the cursor (docs/display.md), so the demo should draw
none and read no input: it becomes a pure client-animation proof whose loop is
independent of the mouse. Removes its cursor layer, mouse subscription, and the
blocking read.

Also harden displayDemoTest to spawn the `input` service alongside the demo
(matching real boot): a client that blocks its animation loop on a mouse read
would now stall before `display-demo: ok` and fail the test, instead of
passing because no input service happened to be present.

Verified: display / display-service / display-demo / display-cursor all green.

system/kernel/tests.zig

@@ -2850,6 +2850,11 @@ fn displayDemoTest(boot_information: *const BootInformation) void {
         result();
         return;
     }
+    // Spawn the input service too — real boot has it, and it guards the demo's
+    // independence from input: the demo must animate to `display-demo: ok` on its own
+    // frame timer even with the input service available (a client that blocks its
+    // animation loop on a mouse read would stall here, never reaching the marker).
+    _ = spawnNamed(rd, "input");
     _ = spawnNamed(rd, "display-demo");
     scheduler.setPriority(1); // below the service + demo, so they run
     while (true) scheduler.yield();