Guarded fixes: repair a file without moving anyone's land

A fix is only useful if you can trust it. The checker separates fixes into two kinds and treats them differently.

Safe fixes

Some problems are about how the file is written, not where the land is: a missing closing point, a quoted Area, a misspelled property name, a byte order mark, a Point in double brackets. These are fixed in one click, Fix N safe problems: and each change is listed in the change log. The area of every plot is measured before and after, and a "safe" fix that changed it would be refused.

Fixes that change a boundary

Untangling a figure eight, removing a fold, cutting out a hole, swapping coordinates, reprojecting: these change geometry. For each one:

1. The proposal is drawn on the map, dashed, over the original.
2. Every affected plot is listed with its area in hectares before and after, and flagged if it crosses the four-hectare line.
3. Nothing changes until you choose Accept the change and apply.
4. Any fix that would change a plot's area by more than 2% is refused, however it is asked. A change that large is a redraw, and it needs someone who knows the field.

Edits, not rewrites

Fixes change the characters they need to and nothing else. Your formatting, property order and number formatting survive, so the change is small enough to review and undo.