display: a ~60 Hz frame clock — presents are scheduled, not immediate

Client 'present' requests and cursor pokes no longer repaint on the spot:
they accumulate damage and arm a one-shot 16 ms timer, and the tick
composites everything pending as one frame. A fast mouse previously
turned every input event into a full present (100+/s); now any number of
draws and moves inside one interval coalesce into a single repaint.

No backend has a real vblank to pace by (docs/display-v2.md, 'Fenced is
not vsync'), so this is the software stand-in — the same strategy Linux
uses atop virtio-gpu. Bring-up paths that need pixels synchronously
(initialise, self-checks) still present directly.

onNotification now handles the message and timer badge bits
independently: one coalesced badge can carry both, and the old
either/or dispatch would have dropped a tick.

All six display QEMU cases pass.

system/services/display/display.zig

@@ -10,7 +10,9 @@
 //! z-order, and visibility. Clients create layers, draw into them by command (`fill_rect`,
 //! `blit_tile`), mark `damage`, and ask for a `present`; the compositor repaints only the
 //! damaged region — clear it, paint the visible layers bottom-to-top into the backend's
-//! surface, then `backend.present(damage)`. Shared-memory client surfaces are later
+//! surface, then `backend.present(damage)`. Presents are paced by a ~60 Hz **frame clock**
+//! (see `schedulePresent`), so any number of client presents and cursor moves inside one
+//! interval coalesce into a single frame. Shared-memory client surfaces are later
 //! (docs/display-v2.md).
 
 const std = @import("std");
@@ -78,6 +80,37 @@ const damage_mode: DamageMode = .grid;
 var damage_list: compositor.DamageList = .{};
 var damage_grid: compositor.TileGrid = .{};
 
+/// The **frame clock**: client `present` requests and cursor motion don't repaint
+/// immediately — they accumulate damage and arm a one-shot timer, and the tick composites
+/// everything pending as one frame. That paces presents to ~60 Hz no matter how fast
+/// clients draw or the mouse moves (previously every mouse event became a full present).
+/// No backend has a real vblank to pace by (docs/display-v2.md, "Fenced is not vsync");
+/// this is the software stand-in, the same strategy Linux uses atop virtio-gpu. Bring-up
+/// paths that need pixels on screen *now* (initialise, the self-checks) still call
+/// `present()` directly.
+const frame_interval_milliseconds = 16;
+var frame_timer_armed = false;
+
+/// Arm the frame clock unless a tick is already pending: any number of requests inside
+/// one interval coalesce into that single tick's present.
+fn schedulePresent() void {
+    if (frame_timer_armed) return;
+    frame_timer_armed = true;
+    _ = system.timerOnce(service_endpoint, frame_interval_milliseconds);
+}
+
+/// A timer landing — the frame clock, or the deferred first native present armed by
+/// `attach_scanout`: present the accumulated damage, then run the one-shot mode-set
+/// self-check if the native upgrade queued it.
+fn frameTick() void {
+    frame_timer_armed = false;
+    present();
+    if (pending_modeset_check) {
+        pending_modeset_check = false;
+        modesetSelfCheck();
+    }
+}
+
 // --- geometry helpers -------------------------------------------------------
 
 fn screenRect() Rect {
@@ -474,15 +507,15 @@ fn mouseListener(width: u32, height: u32) void {
     }
 }
 
-/// Consume the latest cursor position from the channel and repaint the cursor layer at
-/// it. Runs on the main loop (the compositor owner) in response to a listener poke.
-/// `configureLayer` damages both the old and new footprints, so a plain `present`
-/// repaints exactly the two rectangles that changed.
+/// Consume the latest cursor position from the channel and move the cursor layer to it.
+/// Runs on the main loop (the compositor owner) in response to a listener poke.
+/// `configureLayer` damages both the old and new footprints; the frame clock presents
+/// them at the next tick, so a fast mouse coalesces to at most ~60 repaints a second.
 fn renderCursor() void {
     const snapshot = cursor_channel.take() orelse return;
     const id = cursor_layer orelse return;
     _ = configureLayer(id, snapshot.x, snapshot.y, cursor_z, true);
-    present();
+    schedulePresent();
     if (!cursor_tracking_reported and
         @abs(snapshot.x - cursor_origin_x) >= cursor_report_threshold and
         @abs(snapshot.y - cursor_origin_y) >= cursor_report_threshold)
@@ -592,7 +625,9 @@ fn onMessage(message: []const u8, reply: []u8, sender: u32, capability: ?ipc.Han
             return ok(reply);
         },
         @intFromEnum(protocol.Operation.present) => {
-            present();
+            // Scheduled, not immediate: the frame clock composites the accumulated damage
+            // at the next tick, so back-to-back client presents coalesce into one frame.
+            schedulePresent();
             return ok(reply);
         },
         @intFromEnum(protocol.Operation.attach_scanout) => {
@@ -622,21 +657,15 @@ fn onMessage(message: []const u8, reply: []u8, sender: u32, capability: ?ipc.Han
     }
 }
 
-/// Two notification sources reach the compositor. A **message-notification** is a poke
-/// from the mouse-listener thread (a buffered self-`ipc.send`, `notify_message_bit`):
-/// repaint the cursor at its latest channel position. Anything else is the post-attach
-/// present **timer**: repaint into the freshly attached native surface, verify the frame
-/// landed, then run the one-shot mode-set self-check (V5).
+/// Two notification sources reach the compositor, and one coalesced badge can carry
+/// both, so each bit is handled independently. A **message-notification** is a poke from
+/// the mouse-listener thread (a buffered self-`ipc.send`, `notify_message_bit`): fold the
+/// newest cursor position into the scene. A **timer** (`notify_timer_bit`) is the frame
+/// clock — or the deferred first native present after `attach_scanout` — either way,
+/// present the accumulated damage.
 fn onNotification(badge: u64) void {
-    if (badge & ipc.notify_message_bit != 0) {
-        renderCursor();
-        return;
-    }
-    present(); // native present + verify (first timer fire after the upgrade)
-    if (pending_modeset_check) {
-        pending_modeset_check = false;
-        modesetSelfCheck();
-    }
+    if (badge & ipc.notify_message_bit != 0) renderCursor();
+    if (badge & ipc.notify_timer_bit != 0) frameTick();
 }
 
 pub fn main() void {