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-core/src/runtime/menu.rs

@@ -35,6 +35,10 @@ pub enum ActionOutcome {
     NoOp,
 }
 
+/// Maximum number of undo snapshots to retain. Keeps memory
+/// bounded when users toggle hundreds of selections.
+const UNDO_STACK_LIMIT: usize = 50;
+
 /// Tracks multi-select state: which items are selected (by
 /// original index), with undo/redo support.
 pub struct SelectionState {
@@ -57,6 +61,9 @@ impl SelectionState {
     }
 
     fn push_undo(&mut self) {
+        if self.undo_stack.len() >= UNDO_STACK_LIMIT {
+            self.undo_stack.remove(0);
+        }
         self.undo_stack.push(self.selected.clone());
         self.redo_stack.clear();
     }

crates/pikl-test-macros/src/codegen.rs

@@ -141,6 +141,16 @@ fn gen_headless(case: &TestCase, fixtures: &Fixtures) -> syn::Result<TokenStream
         });
     }
 
+    if asserts.is_empty() {
+        return Err(syn::Error::new(
+            case.name.span(),
+            format!(
+                "headless test '{}' has no assertions (expected stdout, stderr contains, or exit)",
+                case.name
+            ),
+        ));
+    }
+
     Ok(quote! {
         #[test]
         fn #test_name() {
@@ -364,8 +374,13 @@ fn gen_menu(case: &TestCase, fixtures: &Fixtures) -> syn::Result<TokenStream> {
             }
         }
     } else {
-        // No assertion on result. Probably an error, but let it compile.
-        quote! {}
+        return Err(syn::Error::new(
+            case.name.span(),
+            format!(
+                "menu test '{}' has no result assertion (expected selected, selected_items, or cancelled)",
+                case.name
+            ),
+        ));
     };
 
     // If test expects cancellation via sender drop (no cancel action, no confirm),

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;
+                        }
                     }
                     _ => {}
                 }

crates/pikl/src/handler.rs

@@ -76,7 +76,9 @@ impl HookHandler for ShellHandlerHook {
         let kind = event.kind();
         if let Some(tx) = self.event_txs.get(&kind) {
             // Non-blocking send. If the channel is full, drop the event.
-            let _ = tx.try_send(event);
+            if let Err(e) = tx.try_send(event) {
+                tracing::debug!(error = %e, "handler event channel full, dropping event");
+            }
         }
         Ok(())
     }
@@ -128,7 +130,13 @@ async fn run_handler_process(
 
     // Writer loop: reads events from channel, writes to stdin
     while let Some(event) = event_rx.recv().await {
-        let json = serde_json::to_string(&event).unwrap_or_default();
+        let json = match serde_json::to_string(&event) {
+            Ok(j) => j,
+            Err(e) => {
+                tracing::warn!(error = %e, "failed to serialize hook event, skipping");
+                continue;
+            }
+        };
         if stdin_writer.write_all(json.as_bytes()).await.is_err() {
             break;
         }

crates/pikl/src/main.rs

@@ -353,8 +353,9 @@ struct CompositeHookHandler {
 
 impl HookHandler for CompositeHookHandler {
     fn handle(&self, event: pikl_core::hook::HookEvent) -> Result<(), PiklError> {
-        // Both fire. Exec is fire-and-forget, handler may
-        // send responses through action_tx.
+        // Exec hooks are fire-and-forget: errors are logged
+        // internally by ShellExecHandler, not propagated here.
+        // Handler hooks are bidirectional, so errors propagate.
         let _ = self.exec.handle(event.clone());
         if let Some(ref h) = self.handler {
             h.handle(event)?;
@@ -507,7 +508,7 @@ fn handle_result(result: Result<MenuResult, PiklError>, cli: &Cli) {
 
 /// Serialize an OutputItem as JSON and write it to the given writer.
 fn write_output_json(writer: &mut impl Write, output: &OutputItem) -> Result<(), std::io::Error> {
-    let json = serde_json::to_string(output).unwrap_or_default();
+    let json = serde_json::to_string(output).map_err(|e| std::io::Error::other(e.to_string()))?;
     writeln!(writer, "{json}")
 }
 
@@ -517,7 +518,8 @@ fn write_plain_value(writer: &mut impl Write, value: &Value) -> Result<(), std::
     match value {
         Value::String(s) => writeln!(writer, "{s}"),
         _ => {
-            let json = serde_json::to_string(value).unwrap_or_default();
+            let json =
+                serde_json::to_string(value).map_err(|e| std::io::Error::other(e.to_string()))?;
             writeln!(writer, "{json}")
         }
     }