perf: fast display (row runs, bulk fills, pixel scroll), terminal draws changed cells only + scrolls pixels, print! batched (was IPC per fmt fragment), snake sleeps on timer

crates/servers/display-server/src/tests/server.rs

@@ -167,3 +167,18 @@ fn text_past_the_edge_is_cut_not_wrapped() {
     server.handle(DisplayRequest::DrawText(low));
     assert_eq!(server.framebuffer().pixel_count(), 256);
 }
+
+#[test]
+fn a_scroll_off_the_screen_moves_nothing() {
+    let mut s = test_server();
+    let scroll = |top| DisplayRequest::Scroll {
+        top: ScreenCoord::new(top),
+        height: Dimension::new(u16::MAX),
+        by: Dimension::new(u16::MAX),
+        fill: Color::from_raw(0),
+    };
+    s.handle(scroll(u16::MAX));
+    assert_eq!(s.framebuffer().scrolls(), 0);
+    s.handle(scroll(0));
+    assert_eq!(s.framebuffer().scrolls(), 1, "clipped, then done");
+}