feat(engine): add platform abstraction trait skeleton (closes #47)

Adds solitaire_engine::platform::{StorageBackend, PlatformTime} traits.
No implementations yet — native and WASM impls follow in #48.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/lib.rs

@@ -31,6 +31,7 @@ pub mod onboarding_plugin;
 pub mod pause_plugin;
 pub mod pending_hint;
 pub mod play_by_seed_plugin;
+pub mod platform;
 pub mod profile_plugin;
 pub mod radial_menu;
 pub mod replay_overlay;
@@ -111,6 +112,7 @@ pub use events::{
 };
 pub use difficulty_plugin::{DifficultyIndexResource, DifficultyPlugin};
 pub use play_by_seed_plugin::{PlayBySeedPlugin, PlayBySeedScreen};
+pub use platform::{PlatformTime, StorageBackend};
 pub use game_plugin::{
     ConfirmNewGameScreen, GameMutation, GameOverScreen, GamePlugin, GameStatePath, RecordingReplay,
     ReplayPath,