T01 primitives: image, divider, backdrop, focus, pressable, measure, tooltip

Ely-built primitives plus gallery demos for every chapter-one entry,
including gpui natives. The gallery now scripts real input by posting
NSEvents to its own queue and shoots each interaction in both modes.
Codex gpt-6-astra (max): three rounds; the last finding, a two-line
doc comment, was fixed after the three-review cap.

src/motion/curve.rs

@@ -1,6 +1,6 @@
 use std::time::Duration;
 
-use gpui::App;
+use gpui::{App, Pixels, px};
 
 use crate::theme::ActiveTheme;
 
@@ -9,6 +9,9 @@ pub const BASE: Duration = Duration::from_millis(200);
 pub const SLOW: Duration = Duration::from_millis(320);
 pub const THEME: Duration = Duration::from_millis(280);
 
+/// Travel of small entrances.
+pub const NUDGE: Pixels = px(4.0);
+
 const DAMPING: f32 = 0.75;
 const OMEGA: f32 = 6.91 / DAMPING;