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.

docs/display.md

@@ -277,11 +277,14 @@ test/qemu_test.py <case>`), each layering on the last:
   layer — logging `display: compositor self-check ok`.
 - **`display-demo`** — the full pipeline from a separate process: the hardware-free
   [`display-demo`](../system/services/display-demo/) client (the
-  [`input-source`](../system/services/input-source/) analog) drives layers — a wallpaper, a
+  [`input-source`](../system/services/input-source/) analog) drives layers — a wallpaper and
-  sliding rectangle, a cursor — through the layer client API and heartbeats
+  a sliding rectangle — through the layer client API and heartbeats
   `display-demo: ok`, proving a frame travelled client → compositor → screen, exactly as
-  the [input test](input.md) proves an event travels source → service → subscriber. The
+  the [input test](input.md) proves an event travels source → service → subscriber. It draws
-  visible motion itself is a screenshot away via `zig build run-x86-64`.
+  no cursor and reads no input — the cursor is the service's own (below), and the demo
+  animates on its own frame timer, independent of the mouse (the test spawns `input`
+  alongside it to keep that independence honest). The visible motion itself is a screenshot
+  away via `zig build run-x86-64`.
 - **`display-cursor`** — the mouse-listener thread end to end: with the `input` service up,
   `input-source mouse` publishes pure motion, and the display's listener thread accumulates
   it into a cursor position handed to the render loop over the `CursorChannel`. Once the

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();

system/services/display-demo/display-demo.zig

@@ -1,15 +1,19 @@
 //! system/services/display-demo — a hardware-free client of the display service, the
-//! `input-source` analog for the compositor. It creates a wallpaper, a rectangle it moves
+//! `input-source` analog for the compositor. It creates a wallpaper and a rectangle it
-//! each frame, and a small cursor, then drives the compositor in a present loop — proof
+//! slides each frame, then drives the compositor in a present loop — proof that a
-//! that a *separate process* can compose a moving scene through the display service over
+//! *separate process* can compose a moving scene through the display service over IPC,
-//! IPC, exercising the layer client API and damage-driven present end to end
+//! exercising the layer client API and damage-driven present end to end
 //! (docs/display.md). It logs `display-demo: ok` once it has driven a run of frames.
+//!
+//! It draws no cursor and reads no input: the on-screen cursor is the display service's
+//! own, tracked by the service's mouse-listener thread (docs/display.md). The demo's job
+//! is only to prove client-driven animation, so its loop runs on its own frame timer and
+//! is deliberately independent of the mouse.
 
 const runtime = @import("runtime");
 const display = runtime.display;
 const system = runtime.system;
 const time = runtime.time;
-const input = runtime.input;
 
 pub fn main() void {
     const mode = display.info() orelse {
@@ -28,15 +32,6 @@ pub fn main() void {
     const box = display.createLayer(0, box_y, box_w, box_h, 1) orelse return createFailed();
     _ = box.fill(0, 0, box_w, box_h, display.color(0xE0, 0x60, 0x40));
 
-    // A little cursor on top. Its position is signed (the layer API is i32) and clamped to
-    // the screen; mouse motion arrives as relative deltas we accumulate below.
-    var cursor_x: i32 = @intCast(mode.width / 2);
-    var cursor_y: i32 = @intCast(mode.height / 2);
-    const cursor_max_x: i32 = @as(i32, @intCast(mode.width)) - 12;
-    const cursor_max_y: i32 = @as(i32, @intCast(mode.height)) - 12;
-    const cursor = display.createLayer(cursor_x, cursor_y, 12, 12, 2) orelse return createFailed();
-    _ = cursor.fill(0, 0, 12, 12, display.color(0xF0, 0xF0, 0xF0));
-
     _ = display.present();
     _ = system.write("display-demo: scene up; animating\n");
 
@@ -45,18 +40,7 @@ pub fn main() void {
     var dx: i32 = 8;
     var frame: u32 = 0;
 
-    var mouse = input.subscribeMouse(); // type: ?input.MouseSubscriber
-    if (mouse == null) _ = system.write("display-demo: no mouse; animating without it\n");
-
     while (true) : (frame += 1) {
-        if (mouse) |*ms| {
-            if (ms.next()) |event| {
-                cursor_x = clamp(cursor_x + event.dx, 0, cursor_max_x);
-                cursor_y = clamp(cursor_y + event.dy, 0, cursor_max_y);
-                _ = cursor.configure(cursor_x, cursor_y, 2, true);
-            }
-        }
-
         x += dx;
         if (x <= 0) {
             x = 0;
@@ -74,13 +58,6 @@ pub fn main() void {
     }
 }
 
-/// Clamp `v` to the inclusive range [lo, hi].
-fn clamp(v: i32, lo: i32, hi: i32) i32 {
-    if (v < lo) return lo;
-    if (v > hi) return hi;
-    return v;
-}
-
 fn createFailed() void {
     _ = system.write("display-demo: create failed\n");
 }