Set Corners: round all, odd-numbered or even-numbered anchors (#5546)

addon.pathtools.cornerSet takes `every` ("all", "odd" or "even"; default
"all"), which keeps, of the anchors already chosen (named, direct-selected
or all), only the odd- or even-numbered ones, counted along each path from
its first anchor as number 1. Anchors that take no corner still count, so
the pattern follows the path's anchor order on open and closed paths; when
nothing is left the command says so. Unknown values are refused.

Object › Plug-ins › Path Precision Tools › Set Corners… (also a button in
the Anchor Inspector's Corners section) opens a dialog with Radius, Corner
kind and Anchors (All / Odd-Numbered / Even-Numbered) that previews on the
canvas and commits as one undo step.

crates/engine/tests/command_sweep.rs

@@ -593,6 +593,9 @@ fn structured_junk() {
         ("addon.pathtools.cornerSet", json!({"items": [1, null, {"anchors": "x"}], "radius": 1})),
         ("addon.pathtools.cornerSet", json!({"ids": [2], "radius": 1e308, "kind": "spiky"})),
         ("addon.pathtools.cornerSet", json!({"items": "all", "radius": f64::NAN, "anchors": [-1, 1.5, "x"]})),
+        ("addon.pathtools.cornerSet", json!({"ids": [2, 3], "radius": 5, "every": "odd"})),
+        ("addon.pathtools.cornerSet", json!({"items": [{"id": 2, "anchors": [1, u64::MAX]}], "radius": 5, "every": "even"})),
+        ("addon.pathtools.cornerSet", json!({"ids": [2], "radius": 5, "every": [1, "x", null]})),
         ("addon.pathtools.cornerRemove", json!({"ids": [0, 1, 2, 3, u64::MAX]})),
         ("addon.pathtools.cornerRemove", json!({"items": [{"id": 2, "anchors": [u64::MAX]}]})),
         ("addon.pathtools.slideAnchor", json!({"id": 2, "anchor": 1, "t": 0.98})),