Problem: in insert mode, `<BS>`, `<C-h>`, `<C-w>`, and `<C-u>` could
delete backwards past the name column boundary into the icon/permissions/ID
prefix, corrupting the line and breaking the parser.
Solution: add buffer-local insert-mode expr keymaps that compute the name
column boundary (cached per line) and return no-op when the cursor is
already at or before it. `<C-u>` emits exactly enough `<BS>` keys to
delete back to the boundary, never past it.
Closes#133