display: layer client API + the display-demo client (D4)

Drive the compositor from a separate process, proving the pipeline end to end.

- runtime.display: a Layer handle (fill / blitTile / configure / damage /
  destroy), createLayer, and a color(r,g,b) helper that caches the mode and
  packs via protocol.pack. Coordinates are signed over the u32 wire fields
  (@bitCast both ways), so a layer may sit or move partly off-screen.
- system/services/display-demo: the input-source analog for the compositor — a
  full-screen wallpaper, a rectangle it slides back and forth (moved by
  configure each frame, so the damage-driven present repaints old + new), and a
  cursor. Presents in a loop paced by runtime.time. Wired into build + initrd.

Fix this surfaced: protocol.message_maximum was 4096, but the kernel caps every
IPC message at MESSAGE_MAXIMUM = 256, so replyWait rejected the oversized receive
buffer with -E2BIG and the serve loop had been spinning since D2 (invisibly, as
those gates matched init-time heartbeats). Set it to 256; blit_tile is now
explicitly a small-tile path (larger bitmaps are the deferred shm surface).

Gate: `python3 test/qemu_test.py display-demo` — the demo drives frames of
motion through the layer client API and logs `display-demo: ok`. Regression:
zig build test, display (D1), display-service (D2/D3), and default zig build.

test/qemu_test.py

@@ -178,6 +178,13 @@ CASES = [
     {"name": "display-service",
      "expect": r"display: online \d+x\d+ pitch \d+[\s\S]*display: presented frame 0[\s\S]*display: compositor self-check ok",
      "fail":   r"display: could not|self-check FAILED|CPU EXCEPTION|KERNEL PANIC"},
+    # Display demo (D4): a separate process (display-demo) drives the compositor over the
+    # layer client API — wallpaper + a moving rectangle + a cursor, presented in a loop.
+    # `display-demo: ok` is printed only after it drove a run of frames of motion through
+    # the service (the visible motion is a screenshot via `zig build run-x86-64`).
+    {"name": "display-demo",
+     "expect": r"display-demo: scene up[\s\S]*display-demo: ok",
+     "fail":   r"display-demo: (no display|create failed)|display: could not|CPU EXCEPTION|KERNEL PANIC"},
     # Monotonic clock (clock() syscall source): calibrated, advancing, never backwards.
     {"name": "clock",
      "expect": r"DANOS-TEST-RESULT: PASS",