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_app/src/main.rs

@@ -11,7 +11,7 @@ use solitaire_engine::{
     FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
     OnboardingPlugin, PausePlugin, ProfilePlugin, ProgressPlugin, SelectionPlugin, SettingsPlugin,
     StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, UiFocusPlugin, UiModalPlugin,
-    WeeklyGoalsPlugin, WinSummaryPlugin,
+    UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
 };
 
 fn main() {
@@ -100,6 +100,7 @@ fn main() {
         .add_plugins(WinSummaryPlugin)
         .add_plugins(UiModalPlugin)
         .add_plugins(UiFocusPlugin)
+        .add_plugins(UiTooltipPlugin)
         .run();
 }