chore(engine): final cleanup after platform abstraction refactor (closes #51)

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

solitaire_engine/src/platform/storage.rs

@@ -1,3 +1,8 @@
+//! Platform-specific persistent storage backends.
+//!
+//! Native builds persist bytes under the app data directory, while browser
+//! builds route the same engine API through `localStorage`.
+
 use std::io;
 use std::sync::Arc;
 

solitaire_engine/src/platform/time.rs

@@ -1,3 +1,5 @@
+//! Platform-specific wall-clock time sources.
+
 /// Abstracts platform-specific wall-clock time.
 ///
 /// Native: backed by `std::time::SystemTime`.