Simplify and Add Anchor Points edited the document whenever the
selection held a path, so a call that changed no path still recorded
an undo step, emptied Redo, marked the document modified and turned
the selected live shapes into plain paths. OK in the Simplify dialog
did the same. Simplify at its default options leaves a rectangle, a
star, a line and a path with one anchor as they are, and Add Anchor
Points leaves a path with one anchor as it is.
Both commands go through edit_paths, which now computes the new paths
before it edits the document. When no path changes, the command
returns without editing the document, and the reply
({ids, before, after}) has the same form and counts as before. When
some paths change, the command replaces only those paths and records
one undo step, so a path that Simplify leaves as it is keeps its live
shape.
One line in the evidence list of docs/target-app-parity.md names the
commands.
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