feat(tui): Phase 2 detail screens — Issue Detail, MR Detail, discussion tree, cross-refs

Implements the remaining Phase 2 Core Screens:

- Discussion tree widget (view/common/discussion_tree.rs): DiscussionNode/NoteNode types,
  expand/collapse state, visual row flattening, format_relative_time with Clock trait
- Cross-reference widget (view/common/cross_ref.rs): CrossRefKind enum, navigable refs,
  badge rendering ([MR]/[REL]/[REF])
- Issue Detail (state + action + view): progressive hydration (metadata Phase 1,
  discussions Phase 2), section cycling, description scroll, sanitized GitLab content
- MR Detail (state + action + view): tab bar (Overview/Files/Discussions), file changes
  with change type indicators, branch info, draft/merge status, diff note support
- Message + update wiring: IssueDetailLoaded, MrDetailLoaded, DiscussionsLoaded handlers
  with TaskSupervisor stale-result guards

Closes bd-1d6z, bd-8ab7, bd-3t1b, bd-1cl9 (Phase 2 epic).
389 tests passing, clippy clean, fmt clean.

crates/lore-tui/src/app/update.rs

@@ -332,6 +332,58 @@ impl LoreApp {
                 Cmd::none()
             }
 
+            // --- Issue detail ---
+            Msg::IssueDetailLoaded {
+                generation,
+                key,
+                data,
+            } => {
+                let screen = Screen::IssueDetail(key.clone());
+                if self
+                    .supervisor
+                    .is_current(&TaskKey::LoadScreen(screen.clone()), generation)
+                {
+                    self.state.issue_detail.apply_metadata(*data);
+                    self.state.set_loading(screen.clone(), LoadState::Idle);
+                    self.supervisor
+                        .complete(&TaskKey::LoadScreen(screen), generation);
+                }
+                Cmd::none()
+            }
+            Msg::DiscussionsLoaded {
+                generation,
+                key,
+                discussions,
+            } => {
+                let screen = Screen::IssueDetail(key.clone());
+                if self
+                    .supervisor
+                    .is_current(&TaskKey::LoadScreen(screen.clone()), generation)
+                {
+                    self.state.issue_detail.apply_discussions(discussions);
+                }
+                Cmd::none()
+            }
+
+            // --- MR detail ---
+            Msg::MrDetailLoaded {
+                generation,
+                key,
+                data,
+            } => {
+                let screen = Screen::MrDetail(key.clone());
+                if self
+                    .supervisor
+                    .is_current(&TaskKey::LoadScreen(screen.clone()), generation)
+                {
+                    self.state.mr_detail.apply_metadata(*data);
+                    self.state.set_loading(screen.clone(), LoadState::Idle);
+                    self.supervisor
+                        .complete(&TaskKey::LoadScreen(screen), generation);
+                }
+                Cmd::none()
+            }
+
             // All other message variants: no-op for now.
             // Future phases will fill these in as screens are implemented.
             _ => Cmd::none(),