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/resources.rs

@@ -0,0 +1,55 @@
+//! Bevy resources owned by the engine crate.
+
+use bevy::math::Vec2;
+use bevy::prelude::Resource;
+use chrono::{DateTime, Utc};
+use solitaire_core::game_state::GameState;
+use solitaire_core::pile::PileType;
+
+/// Wraps the currently active `GameState`. Single source of truth for the in-progress game.
+#[derive(Resource, Debug, Clone)]
+pub struct GameStateResource(pub GameState);
+
+/// Tracks an in-progress drag operation.
+///
+/// When `cards` is empty there is no active drag. When non-empty, the listed cards
+/// are being moved by the user and should be rendered at the cursor position.
+#[derive(Resource, Debug, Clone, Default)]
+pub struct DragState {
+    pub cards: Vec<u32>,
+    pub origin_pile: Option<PileType>,
+    pub cursor_offset: Vec2,
+    pub origin_z: f32,
+}
+
+impl DragState {
+    /// Returns true when no drag is currently in progress.
+    pub fn is_idle(&self) -> bool {
+        self.cards.is_empty()
+    }
+
+    /// Clears the drag state.
+    pub fn clear(&mut self) {
+        self.cards.clear();
+        self.origin_pile = None;
+        self.cursor_offset = Vec2::ZERO;
+        self.origin_z = 0.0;
+    }
+}
+
+/// Current sync activity — shown in the settings screen.
+///
+/// Defined here rather than in `solitaire_data` because it is a UI/runtime
+/// status value, not part of the persistence layer.
+#[derive(Debug, Clone, Default)]
+pub enum SyncStatus {
+    #[default]
+    Idle,
+    Syncing,
+    LastSynced(DateTime<Utc>),
+    Error(String),
+}
+
+/// Bevy resource wrapping the current `SyncStatus`.
+#[derive(Resource, Debug, Clone, Default)]
+pub struct SyncStatusResource(pub SyncStatus);