feat(engine): add weekly goals with ISO-week rollover and +75 XP bonus

Phase 6 part 2b:
- solitaire_data::weekly defines WeeklyGoalKind, WeeklyGoalDef,
  WeeklyGoalContext, current_iso_week_key, and three starter goals
  (5 wins, 3 no-undo wins, 3 fast wins).
- PlayerProgress gains weekly_goal_week_iso, roll_weekly_goals_if_new_week,
  and record_weekly_progress (returns true exactly once per goal completion).
- WeeklyGoalsPlugin evaluates GameWonEvent against WEEKLY_GOALS, rolls the
  week if needed, increments matching counters, awards WEEKLY_GOAL_XP for
  newly-completed goals, persists progress, and fires
  WeeklyGoalCompletedEvent.

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

solitaire_app/src/main.rs

@@ -1,7 +1,7 @@
 use bevy::prelude::*;
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, CardPlugin, DailyChallengePlugin, GamePlugin, InputPlugin,
-    ProgressPlugin, StatsPlugin, TablePlugin,
+    ProgressPlugin, StatsPlugin, TablePlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -25,5 +25,6 @@ fn main() {
         .add_plugins(ProgressPlugin::default())
         .add_plugins(AchievementPlugin::default())
         .add_plugins(DailyChallengePlugin)
+        .add_plugins(WeeklyGoalsPlugin)
         .run();
 }