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/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>>;
+}