feat(data): add PlayerProgress with XP/level helpers and atomic persistence

level_for_xp implements the two-segment level formula from
ARCHITECTURE.md §13. xp_for_win = base 50 + linearly-scaled speed bonus
(10..=50 for sub-2-minute wins) + 25 if no undo was used. PlayerProgress
exposes add_xp returning the previous level so callers can detect
level-up events.

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

solitaire_data/src/lib.rs

@@ -45,3 +45,9 @@ pub mod achievements;
 pub use achievements::{
     achievements_file_path, load_achievements_from, save_achievements_to, AchievementRecord,
 };
+
+pub mod progress;
+pub use progress::{
+    level_for_xp, load_progress_from, progress_file_path, save_progress_to, xp_for_win,
+    PlayerProgress,
+};