DRY pass #3: checklist near-misses live in the grammar fixture (#5373)

test_notes.py and derive.rs each kept the same six lines of prose that look
like a checklist item. Four were already task_lines cases in grammar.json;
the other two (empty brackets, no bullet) join them, so the web suite now
tests them too, and both hand lists go.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/testdata/grammar.json

@@ -26,6 +26,8 @@
     { "line": "+ [ ] plus bullet", "item": null },
     { "line": "- [y] x", "item": null },
     { "line": "- [  ] x", "item": null },
+    { "line": "- [] empty brackets", "item": null },
+    { "line": "[ ] no bullet at all", "item": null },
     { "line": "text - [ ] x", "item": null },
     { "line": "- milk", "item": null },
     { "line": "", "item": null }