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/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;
+}