feat(engine): tooltip infrastructure with hover delay (foundation only)

A new ui_tooltip module owns a Tooltip(Cow<'static, str>) component
that turns any UI node into a hover-revealing help target. Bevy 0.18's
required-components attribute auto-inserts an Interaction so callers
just attach Tooltip and the rest is wired.

A single overlay entity is reparented above the focus ring (new
Z_TOOLTIP token = Z_FOCUS_RING + 10) and tracked from the hovered
target's GlobalTransform + ComputedNode. The chained Update systems
start a hover timer on Interaction::Hovered, show the overlay once
MOTION_TOOLTIP_DELAY_SECS (0.5s) has elapsed, hide it the moment hover
ends, and refresh the text when the hover target switches without an
intervening unhover.

Tested headless under MinimalPlugins with a 200ms ManualDuration
ticker — Bevy clamps Time<Virtual>'s max_delta to 250ms by default, so
a one-shot 1s step doesn't actually advance the clock past the
threshold; the tests step five times to exercise both pre- and
post-delay invariants.

This commit ships the infrastructure only — no entity in the engine
has Tooltip attached yet. A follow-up applies tooltips to the HUD
readouts and action bar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/ui_theme.rs

@@ -275,6 +275,23 @@ pub const MOTION_SCORE_PULSE_SECS: f32 = 0.25;
 /// 400 ms.
 pub const MOTION_LOADING_TICK_SECS: f32 = 0.40;
 
+/// Hover delay before a tooltip appears, in seconds. Long enough that
+/// players gliding the cursor across the HUD don't see flicker; short
+/// enough that "stop and read" feels responsive. Not run through
+/// [`scaled_duration`] — `AnimSpeed` controls gameplay motion, not the
+/// hover-discoverability budget for help text.
+pub const MOTION_TOOLTIP_DELAY_SECS: f32 = 0.5;
+
+// ---------------------------------------------------------------------------
+// Z-index — tooltip layer
+// ---------------------------------------------------------------------------
+
+/// Z-layer for tooltips. Sits one rung above the focus ring so a
+/// tooltip rendered over a focused button is never occluded by the
+/// button's outline. Still below `Z_WIN_CASCADE` and `Z_TOAST` so the
+/// celebration and notification layers stay on top.
+pub const Z_TOOLTIP: i32 = Z_FOCUS_RING + 10;
+
 // ---------------------------------------------------------------------------
 // Helpers
 // ---------------------------------------------------------------------------
@@ -342,6 +359,7 @@ mod tests {
             Z_PAUSE_DIALOG,
             Z_ONBOARDING,
             Z_FOCUS_RING,
+            Z_TOOLTIP,
             Z_WIN_CASCADE,
             Z_TOAST,
         ];