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-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),