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,

solitaire_engine/src/platform/mod.rs

@@ -0,0 +1,11 @@
+//! Platform abstraction layer.
+//!
+//! Traits defined here are implemented by:
+//! - `solitaire_data` for native targets (filesystem, `std::time`)
+//! - future WASM-specific impls for browser targets (`localStorage`, `js_sys::Date`)
+
+pub mod storage;
+pub mod time;
+
+pub use storage::StorageBackend;
+pub use time::PlatformTime;

solitaire_engine/src/platform/storage.rs

@@ -0,0 +1,19 @@
+use std::io;
+
+/// Abstracts platform-specific key-value / file storage.
+///
+/// Native: backed by the filesystem (via `solitaire_data`).
+/// WASM: backed by `localStorage` / `sessionStorage`.
+pub trait StorageBackend: Send + Sync + 'static {
+    /// Read bytes for the given key. Returns `None` if the key does not exist.
+    fn read(&self, key: &str) -> io::Result<Option<Vec<u8>>>;
+
+    /// Write bytes for the given key atomically.
+    fn write(&self, key: &str, data: &[u8]) -> io::Result<()>;
+
+    /// Delete a key. No-op if the key does not exist.
+    fn delete(&self, key: &str) -> io::Result<()>;
+
+    /// List all known keys (for migration / debug purposes).
+    fn keys(&self) -> io::Result<Vec<String>>;
+}

solitaire_engine/src/platform/time.rs

@@ -0,0 +1,11 @@
+/// Abstracts platform-specific wall-clock time.
+///
+/// Native: backed by `std::time::SystemTime`.
+/// WASM: backed by `js_sys::Date::now()`.
+pub trait PlatformTime: Send + Sync + 'static {
+    /// Returns the current Unix timestamp in seconds.
+    fn now_unix_secs(&self) -> u64;
+
+    /// Returns the current Unix timestamp in milliseconds.
+    fn now_unix_millis(&self) -> u128;
+}