docs(engine): add missing doc comments on layout, ProgressPlugin; fix audio format in ARCHITECTURE.md

- Add field-level doc comments to Layout::card_size and Layout::pile_positions
- Add struct-level doc comment to ProgressPlugin
- Fix ARCHITECTURE.md Section 14: .ogg → .wav throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/progress_plugin.rs

@@ -37,6 +37,11 @@ pub struct LevelUpEvent {
 #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
 pub struct ProgressUpdate;
 
+/// Bevy plugin that awards XP on `GameWonEvent`, persists `PlayerProgress`,
+/// and emits `LevelUpEvent` whenever a win crosses a level boundary.
+///
+/// Use `ProgressPlugin::default()` in the main app (reads/writes the platform
+/// data directory) and `ProgressPlugin::headless()` in tests (no I/O).
 pub struct ProgressPlugin {
     pub storage_path: Option<PathBuf>,
 }