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/lib.rs

@@ -33,6 +33,7 @@ pub mod time_attack_plugin;
 pub mod ui_focus;
 pub mod ui_modal;
 pub mod ui_theme;
+pub mod ui_tooltip;
 pub mod weekly_goals_plugin;
 pub mod win_summary_plugin;
 
@@ -104,6 +105,7 @@ pub use ui_modal::{
     spawn_modal_header, ButtonVariant, ModalActions, ModalBody, ModalButton, ModalCard,
     ModalHeader, ModalScrim, UiModalPlugin,
 };
+pub use ui_tooltip::{Tooltip, UiTooltipPlugin};
 pub use table_plugin::{
     BackgroundImageSet, HintPileHighlight, PileMarker, TableBackground, TablePlugin,
 };