feat(tui): wire entity cache for near-instant detail view reopens (bd-3rjw)

- Add get_mut() and clear() methods to EntityCache<V>
- Add CachedIssuePayload / CachedMrPayload types to state
- Wire cache check in navigate_to for instant cache hits
- Populate cache on IssueDetailLoaded / MrDetailLoaded
- Update cache on DiscussionsLoaded
- Add 6 new entity_cache tests (get_mut, clear)

crates/lore-tui/src/view/file_history.rs

@@ -371,7 +371,11 @@ fn render_mr_list(
 
     // Inline discussion snippets (rendered beneath MRs when toggled on).
     if state.show_discussions && !result.discussions.is_empty() {
-        let visible_mrs = result.merge_requests.len().saturating_sub(offset).min(height);
+        let visible_mrs = result
+            .merge_requests
+            .len()
+            .saturating_sub(offset)
+            .min(height);
         let disc_start_y = start_y + visible_mrs as u16;
         let remaining = height.saturating_sub(visible_mrs);
         render_discussions(frame, result, x, disc_start_y, max_x, remaining, bp);