refactor: Pre-phase-6 review cleanup.

- Propagate serialization errors in output path
instead of unwrap_or_default() silently writing
blank lines (main.rs, handler.rs).
- Cap selection undo stack at 50 entries to bound
memory on heavy multi-select usage (menu.rs).
- Break TUI event loop on dead action channel
instead of silently dropping keypresses (lib.rs).
- Log dropped handler hook events when channel is
full (handler.rs).
- Reject menu and headless DSL tests that have no
assertions at compile time (codegen.rs).
- Document filter_text ownership design in TUI and
exec/handler error asymmetry in CompositeHandler.

crates/pikl-tui/src/lib.rs

@@ -71,12 +71,21 @@ async fn run_inner(
     // Send initial resize
     let size = terminal.size()?;
     let list_height = size.height.saturating_sub(1);
-    let _ = action_tx
+    if action_tx
         .send(Action::Resize {
             height: list_height,
         })
-        .await;
+        .await
+        .is_err()
+    {
+        tracing::warn!("core channel closed before TUI started");
+        return Ok(());
+    }
 
+    // Local copy of filter text for immediate cursor positioning.
+    // Synced from core on every StateChanged. Core is the source
+    // of truth: if IPC or a hook changes the filter, the next
+    // broadcast overwrites this.
     let mut filter_text = String::new();
     let mut view_state: Option<ViewState> = None;
     let mut event_stream = EventStream::new();
@@ -124,12 +133,16 @@ async fn run_inner(
                                 mode = *m;
                                 pending = PendingKey::None;
                             }
-                            let _ = action_tx.send(action).await;
+                            if action_tx.send(action).await.is_err() {
+                                break; // core is gone
+                            }
                         }
                     }
                     Event::Resize(_, h) => {
                         let list_height = h.saturating_sub(1);
-                        let _ = action_tx.send(Action::Resize { height: list_height }).await;
+                        if action_tx.send(Action::Resize { height: list_height }).await.is_err() {
+                            break;
+                        }
                     }
                     _ => {}
                 }