Add AtomLayout::selectable for opt-in text selection (#8224)

* Closes https://github.com/emilk/egui/issues/8217
* [x] I have followed the instructions in the PR template

## What

`AllocatedAtomLayout::paint` painted text via
`ui.painter().galley(...)`, so text in atom-based widgets could never be
selected. As discussed in #8217, routing it through
`LabelSelectionState::label_text_selection` unconditionally would break
`Button` / `Checkbox` / `RadioButton` / `TextEdit` (whose labels should
not be selectable, and whose click/drag handling would fight the
selection drag).

So this adds an **opt-in** `AtomLayout::selectable(bool)` (default
`false`). When enabled, the layout also senses clicks and drags
(mirroring `Label::layout_in_ui`) and paints its text through the
label-selection machinery. The underline is `Stroke::NONE`, so when
nothing is selected the painted output is identical to the existing path
— the default (`selectable == false`) behaviour, and existing snapshots,
are unchanged.

## Tests

Two tests in `tests/egui_tests/tests/test_atoms.rs`:

- `test_atom_selectable_senses_click_and_drag` — a `selectable(true)`
layout senses click+drag; the default stays inert.
- `test_atom_selectable_text_can_be_copied` — selecting (drag) the text
of a selectable layout and copying yields the text via
`OutputCommand::CopyText`, while a non-selectable layout yields nothing.

## Notes

- Verified locally: `cargo fmt --all -- --check`, `cargo clippy -p egui`
(clean), and the two new tests pass. I haven't run the full
`./scripts/check.sh` (wasm/typos) locally, hence opening as a draft.
- This only adds the opt-in API; no existing widget is made selectable.
Happy to wire it up on a specific widget and/or add an `egui_demo_lib`
demo if you'd like — just let me know.

tests/egui_tests/tests/test_atoms.rs

@@ -140,3 +140,94 @@ fn test_atom_letter_spacing() {
 
     harness.snapshot("atom_letter_spacing");
 }
+
+/// `AtomLayout::selectable(true)` should opt the layout into click+drag sensing
+/// so its text can be selected, while the default layout stays inert.
+/// See <https://github.com/emilk/egui/issues/8217>.
+#[test]
+fn test_atom_selectable_senses_click_and_drag() {
+    use egui::{AtomLayout, Sense};
+
+    let mut captured = (Sense::hover(), Sense::hover());
+    {
+        let mut harness = HarnessBuilder::default().build_ui(|ui| {
+            let selectable = AtomLayout::new("selectable").selectable(true).show(ui);
+            let default = AtomLayout::new("default").show(ui);
+            captured = (selectable.response.sense, default.response.sense);
+        });
+        harness.run();
+    }
+
+    let (selectable_sense, default_sense) = captured;
+    assert!(
+        selectable_sense.senses_click() && selectable_sense.senses_drag(),
+        "a selectable AtomLayout should sense clicks and drags"
+    );
+    assert!(
+        !default_sense.senses_drag(),
+        "a non-selectable AtomLayout should stay inert"
+    );
+}
+
+/// Selecting the text of a `selectable` [`egui::AtomLayout`] and copying it should
+/// yield the text, while a non-selectable one yields nothing.
+/// See <https://github.com/emilk/egui/issues/8217>.
+#[test]
+fn test_atom_selectable_text_can_be_copied() {
+    use egui::{AtomLayout, Event, Modifiers, OutputCommand, PointerButton, Pos2, Rect};
+    use std::cell::Cell;
+
+    fn copied_text(selectable: bool) -> Option<String> {
+        let rect_cell = Cell::new(Rect::NOTHING);
+        let mut harness = HarnessBuilder::default()
+            .with_size(Vec2::new(400.0, 100.0))
+            .build_ui(|ui| {
+                let response = AtomLayout::new("selectable atoms")
+                    .selectable(selectable)
+                    .show(ui);
+                rect_cell.set(response.response.rect);
+            });
+        harness.run();
+
+        let rect = rect_cell.get();
+        let left = Pos2::new(rect.left() + 1.0, rect.center().y);
+        let right = Pos2::new(rect.right() - 1.0, rect.center().y);
+
+        // Press at the start of the text and drag to the end to select all of it.
+        harness.event(Event::PointerMoved(left));
+        harness.event(Event::PointerButton {
+            pos: left,
+            button: PointerButton::Primary,
+            pressed: true,
+            modifiers: Modifiers::NONE,
+        });
+        harness.run();
+        harness.event(Event::PointerMoved(right));
+        harness.run();
+
+        // Copy, then read back the clipboard command produced by this frame.
+        harness.event(Event::Copy);
+        harness.step();
+
+        harness
+            .output()
+            .platform_output
+            .commands
+            .iter()
+            .find_map(|cmd| match cmd {
+                OutputCommand::CopyText(text) => Some(text.clone()),
+                _ => None,
+            })
+    }
+
+    assert_eq!(
+        copied_text(true).as_deref(),
+        Some("selectable atoms"),
+        "selectable atom text should be copyable after selecting it"
+    );
+    assert_eq!(
+        copied_text(false),
+        None,
+        "non-selectable atom text should not be selectable"
+    );
+}