Adds per-file cursor position persistence (MC ~/.mc/filepos parity).
Storage: ~/.config/tlc/filepos as a tab-separated canonical-path database. Wired into editor/viewer open+close in both standalone binaries and the in-TLC file manager. CursorPos struct, save/load functions, restore_cursor_position() and save_cursor_position() methods on Editor and Viewer.
buttonbar.rs: add module-level doc to satisfy missing_docs lint.
vasilito
d6aaf4e8af