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/primitives/backdrop.rs

@@ -0,0 +1,83 @@
+use std::rc::Rc;
+
+use gpui::{
+    Animation, AnimationExt, AnyElement, App, ElementId, InteractiveElement, IntoElement,
+    MouseButton, ParentElement, Point, RenderOnce, Styled, Window, anchored, deferred, div,
+    prelude::*,
+};
+use smallvec::SmallVec;
+
+use crate::{motion, theme::ActiveTheme};
+
+type Dismiss = Rc<dyn Fn(&mut Window, &mut App)>;
+
+/// Full-window scrim. Children sit centered above it.
+#[derive(IntoElement)]
+pub struct Backdrop {
+    id: ElementId,
+    on_dismiss: Option<Dismiss>,
+    children: SmallVec<[AnyElement; 1]>,
+}
+
+impl Backdrop {
+    pub fn new(id: impl Into<ElementId>) -> Self {
+        Self {
+            id: id.into(),
+            on_dismiss: None,
+            children: SmallVec::new(),
+        }
+    }
+
+    /// Runs on a click outside the children.
+    pub fn on_dismiss(mut self, handler: impl Fn(&mut Window, &mut App) + 'static) -> Self {
+        self.on_dismiss = Some(Rc::new(handler));
+        self
+    }
+}
+
+impl ParentElement for Backdrop {
+    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
+        self.children.extend(elements);
+    }
+}
+
+impl RenderOnce for Backdrop {
+    fn render(self, window: &mut Window, cx: &mut App) -> impl IntoElement {
+        let viewport = window.viewport_size();
+        let fade = motion::duration(motion::BASE, cx);
+        let scrim = div()
+            .id(self.id.clone())
+            .absolute()
+            .top_0()
+            .left_0()
+            .size_full()
+            .bg(cx.theme().colors.backdrop)
+            .occlude()
+            .when_some(self.on_dismiss, |scrim, dismiss| {
+                scrim.on_mouse_down(MouseButton::Left, move |_, window, cx| dismiss(window, cx))
+            })
+            .with_animation(
+                self.id,
+                Animation::new(fade).with_easing(motion::ease_out_cubic),
+                |scrim, t| scrim.opacity(t),
+            );
+
+        deferred(
+            anchored().position(Point::default()).child(
+                div()
+                    .relative()
+                    .w(viewport.width)
+                    .h(viewport.height)
+                    .flex()
+                    .items_center()
+                    .justify_center()
+                    .child(scrim)
+                    .children(
+                        self.children
+                            .into_iter()
+                            .map(|child| div().relative().occlude().child(child)),
+                    ),
+            ),
+        )
+    }
+}