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();
     }