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_data/src/lib.rs

@@ -51,3 +51,9 @@ pub use progress::{
     daily_seed_for, level_for_xp, load_progress_from, progress_file_path, save_progress_to,
     xp_for_win, PlayerProgress,
 };
+
+pub mod weekly;
+pub use weekly::{
+    current_iso_week_key, weekly_goal_by_id, WeeklyGoalContext, WeeklyGoalDef, WeeklyGoalKind,
+    WEEKLY_GOALS, WEEKLY_GOAL_XP,
+};