feat(engine): add resources, events, and GamePlugin event routing

Introduces the plumbing layer for Phase 3: GameStateResource wraps
solitaire_core::GameState, DragState tracks in-progress drags, and
SyncStatusResource holds runtime sync status. GamePlugin routes
Draw/Move/Undo/NewGame request events into GameState and emits
StateChangedEvent and GameWonEvent for downstream systems.

Also adds the Phase 3 implementation plan under docs/superpowers/plans/.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/events.rs

@@ -0,0 +1,43 @@
+//! Cross-system events used by the engine's plugins.
+
+use bevy::prelude::Event;
+use solitaire_core::pile::PileType;
+
+/// Request to move `count` cards from `from` to `to`. Fired by input systems,
+/// consumed by `GamePlugin`.
+#[derive(Event, Debug, Clone)]
+pub struct MoveRequestEvent {
+    pub from: PileType,
+    pub to: PileType,
+    pub count: usize,
+}
+
+/// Request to draw from the stock (or recycle waste when stock is empty).
+#[derive(Event, Debug, Clone, Copy, Default)]
+pub struct DrawRequestEvent;
+
+/// Request to undo the most recent state change.
+#[derive(Event, Debug, Clone, Copy, Default)]
+pub struct UndoRequestEvent;
+
+/// Request to start a new game. `seed = None` uses a system-time seed.
+#[derive(Event, Debug, Clone, Copy, Default)]
+pub struct NewGameRequestEvent {
+    pub seed: Option<u64>,
+}
+
+/// Fired by `GamePlugin` after any successful state mutation. Rendering and
+/// score-display systems listen for this to refresh.
+#[derive(Event, Debug, Clone, Copy, Default)]
+pub struct StateChangedEvent;
+
+/// Fired once when the active game transitions to won.
+#[derive(Event, Debug, Clone, Copy)]
+pub struct GameWonEvent {
+    pub score: i32,
+    pub time_seconds: u64,
+}
+
+/// Fired when a card's face-up state changes during gameplay.
+#[derive(Event, Debug, Clone, Copy)]
+pub struct CardFlippedEvent(pub u32);