shell: add spotlight builtin (theme 04), dots/placeholder-clock schema, and anim::spring_to

- bread-theme:🐚 WorkspacesModule.dot_widths and ClockModule.placeholder_clock,
  resolved/validated in manifest.rs and modeled in types.rs, so a theme can
  actually configure dot-pill widths and an entry-as-clock placeholder
  instead of those staying schema-only.
- New compiled-in builtin "spotlight" (assets/shell/spotlight/), registered
  in builtin.rs so list() returns three themes: centred capsule window spec
  (anchors=[top] only, width=480, exclusive=none, keyboard=on_demand),
  workspaces.style=dots, clock.style=none+placeholder_clock, launcher.mode=
  embedded, drawer=[launcher_results]. Colours are palette token names
  (pink), never hex.
- bread-theme::anim::spring_to: a reusable TickCallback size-interpolation
  helper (GTK4 has no CSS width/height transition on a widget or layer-shell
  surface), generalized from WorkspaceTrail's existing ease/tick pattern, for
  breadbar's capsule-drawer expand/collapse.
- bread_launcher::LAUNCHER_APP: the launcher's one shared on-disk identity
  (cache/config/history), so breadbar's embedded capsule and breadbox's
  overlay window read and write the SAME cache and launch history rather
  than forking into two rankings of a user's apps.

Tests: +6 spotlight builtin tests (loads/in list/capsule window shape/
embedded mode/dots widths/flat-pink-not-hex), bread-theme --lib 63->69,
bread-launcher --lib unchanged at 18.

bread-theme/src/anim.rs

@@ -0,0 +1,68 @@
+//! `anim::spring_to` — THEME_SYSTEM_PLAN.md §7/§8: GTK4 has no CSS
+//! width/height transition on a widget or a layer-shell surface (unlike the
+//! demos' `transition: width .45s var(--spring)` /
+//! `transition: max-height .4s var(--spring)`), so a size change that wants
+//! to animate has to interpolate a plain integer over the frame clock
+//! instead and re-apply it (`set_size_request`, `set_default_width`, ...)
+//! every frame.
+//!
+//! This is the same house technique `breadbar::bar::workspaces::WorkspaceTrail`
+//! already uses for the workspace trail's stretch/snap (a local, un-exported
+//! `ease`/`ease_overshoot` pair driven by `add_tick_callback` and
+//! `Instant::elapsed`) — lifted here, generalized to a plain `i32 -> i32`
+//! interpolation with a caller-supplied frame callback, so theme 04's
+//! capsule-drawer expand/collapse (and any future popover that wants the
+//! same effect) doesn't have to reimplement it.
+
+use gtk4::glib::ControlFlow;
+use gtk4::prelude::*;
+
+/// Approximates `cubic-bezier(0.22, 1.35, 0.36, 1)` — the overshoot/"spring"
+/// curve every builtin theme's `tokens.spring` names (`Tokens::spring`'s own
+/// default). Not a literal bezier solve (same approximation
+/// `bar::workspaces::ease_overshoot` uses) — exact enough that the eye can't
+/// tell it apart from the real curve at animation speeds.
+fn spring_ease(t: f64) -> f64 {
+    let t = t.clamp(0.0, 1.0);
+    let c = 1.35;
+    let t1 = t - 1.0;
+    1.0 + t1 * t1 * ((c + 1.0) * t1 + c)
+}
+
+/// Interpolates from `from` to `to` over `duration_ms`, calling `on_frame`
+/// with each intermediate value (and, on the final tick, the exact `to` —
+/// never an off-by-rounding near-miss) via `widget`'s frame clock.
+///
+/// Returns the [`gtk4::TickCallbackId`] so a caller that might need to
+/// interrupt an in-flight run (e.g. the drawer re-opening before its close
+/// animation finished) can `.remove()` it early; a run left to finish on its
+/// own needs no cleanup — the callback self-terminates by returning
+/// [`ControlFlow::Break`] once `duration_ms` has elapsed, same as
+/// `WorkspaceTrail`'s own tick callbacks.
+pub fn spring_to(
+    widget: &impl IsA<gtk4::Widget>,
+    from: i32,
+    to: i32,
+    duration_ms: f64,
+    on_frame: impl FnMut(i32) + 'static,
+) -> gtk4::TickCallbackId {
+    let started = std::time::Instant::now();
+    // `add_tick_callback` requires `Fn`, not `FnMut` — the caller's frame
+    // closure almost always needs to mutate captured state (a widget's size
+    // request, an `Rc<Cell<..>>` flag), so it's boxed behind a `RefCell`
+    // here rather than pushing `Cell`/`RefCell` plumbing onto every call
+    // site (every current and future caller wants `FnMut`, none want `Fn`).
+    let on_frame = std::cell::RefCell::new(on_frame);
+    widget.add_tick_callback(move |_, _| {
+        let elapsed = started.elapsed().as_secs_f64() * 1000.0;
+        let mut on_frame = on_frame.borrow_mut();
+        if elapsed >= duration_ms {
+            on_frame(to);
+            return ControlFlow::Break;
+        }
+        let t = spring_ease(elapsed / duration_ms);
+        let value = from as f64 + (to - from) as f64 * t;
+        on_frame(value.round() as i32);
+        ControlFlow::Continue
+    })
+}