- implement full `PenMode.line` flow (start/update/finish), rendering, erase hit-testing, and persistence/JSON/Hive support
- extend traversal-time overlays to support both free-draw paths and straight lines
- add shortcuts for save (`Ctrl/Cmd+S`), add page (`C`), and toggle lineup (`G`) with UI shortcut hints
- include provider tests for drawing serialization/integrity updates