fix(engine,gpgs,core,server): export CardFaceRevealedEvent, explicit gpgs stub, enum/constant docs

- engine/lib.rs: re-export CardFaceRevealedEvent so external crates can consume flip-midpoint audio events
- gpgs/stub.rs: add explicit impls for all six defaulted SyncProvider methods; future trait changes now cause a compile error in the stub rather than silently picking up wrong defaults
- core/game_state.rs: add /// doc comments to DrawMode and GameMode variants
- server/auth.rs: replace terse BCRYPT_COST comment with full /// doc comment matching ARCHITECTURE.md §19
- server/leaderboard.rs: add /// doc comment to DISPLAY_NAME_MAX; fix misplaced comment that was prepended to the opt_in handler instead of the constant

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -64,9 +64,10 @@ pub use card_plugin::{
 };
 pub use cursor_plugin::CursorPlugin;
 pub use events::{
-    AchievementUnlockedEvent, CardFlippedEvent, DrawRequestEvent, ForfeitEvent, GameWonEvent,
-    HintVisualEvent, InfoToastEvent, ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent,
-    NewGameConfirmEvent, NewGameRequestEvent, StateChangedEvent, UndoRequestEvent, XpAwardedEvent,
+    AchievementUnlockedEvent, CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent,
+    ForfeitEvent, GameWonEvent, HintVisualEvent, InfoToastEvent, ManualSyncRequestEvent,
+    MoveRejectedEvent, MoveRequestEvent, NewGameConfirmEvent, NewGameRequestEvent,
+    StateChangedEvent, UndoRequestEvent, XpAwardedEvent,
 };
 pub use game_plugin::{ConfirmNewGameScreen, GameMutation, GameOverScreen, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};