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/divider.rs

@@ -0,0 +1,58 @@
+use gpui::{App, IntoElement, ParentElement, RenderOnce, SharedString, Styled, Window, div};
+
+use crate::theme::{ActiveTheme, TextSize};
+
+/// Hairline rule, across or down.
+#[derive(IntoElement)]
+pub struct Divider {
+    vertical: bool,
+    label: Option<SharedString>,
+}
+
+impl Divider {
+    pub fn horizontal() -> Self {
+        Self {
+            vertical: false,
+            label: None,
+        }
+    }
+
+    pub fn vertical() -> Self {
+        Self {
+            vertical: true,
+            label: None,
+        }
+    }
+
+    /// Text centered on a horizontal rule.
+    pub fn label(mut self, label: impl Into<SharedString>) -> Self {
+        self.label = Some(label.into());
+        self
+    }
+}
+
+impl RenderOnce for Divider {
+    fn render(self, _: &mut Window, cx: &mut App) -> impl IntoElement {
+        let theme = cx.theme();
+        let line = || div().border_color(theme.colors.border);
+        if self.vertical {
+            return line().h_full().border_l_1();
+        }
+        match self.label {
+            None => line().w_full().border_t_1(),
+            Some(label) => div()
+                .flex()
+                .items_center()
+                .gap_3()
+                .w_full()
+                .child(line().flex_1().border_t_1())
+                .child(
+                    div()
+                        .text_size(theme.text_size(TextSize::Xs))
+                        .text_color(theme.colors.fg_subtle)
+                        .child(label),
+                )
+                .child(line().flex_1().border_t_1()),
+        }
+    }
+}