Offset of an irregular closed spline no longer fails its error certificate; offset fit nodes stay private (#1009)

Fixes #971. At the node limit the offset certificate splits the fitted curve where it meets the exact offset midpoint, so curves with different parameter speeds certify within the same 0.002 mm tolerance. Associated offset fit nodes are hidden from display, picking, box selection and snapping until Break Association.

Thanks @Karl-VonRichter.

crates/ui-egui/tests/scenarios/sketch_offset_irregular_spline.json

@@ -0,0 +1,22 @@
+[
+ {"note": "#971: a 2 mm offset of this closed irregular fit spline used to exhaust its node limit with an endpoint certificate error. Preview must succeed, Cancel must preserve the source, and OK must commit the preview."},
+ {"call": "engine.execute", "params": {"command": "sketch.create", "params": {"plane": "XY"}}},
+ {"call": "engine.execute", "params": {"command": "sketch.spline.fit_point", "params": {"points": [[-17,15],[0,8],[17,13],[15,0],[5,-7],[-1,-14],[-9,-1],[-21,-1]], "periodic": true}}},
+ {"call": "ui.view", "params": {"view": "top"}},
+ {"start": "sketch.offset"},
+ {"click": {"sketch": [0,8]}},
+ {"until": {"count": {"name": "sketch_preview_curves", "value": 1}}},
+ {"expect": {"dialog": "Offset", "sketch_curves": {"own": 1}, "errors": 0}},
+ {"key": "Escape"},
+ {"expect": {"dialog": null, "sketch_curves": {"own": 1}, "errors": 0}},
+ {"start": "sketch.offset"},
+ {"click": {"sketch": [0,8]}},
+ {"until": {"count": {"name": "sketch_preview_curves", "value": 1}}},
+ {"key": "Enter"},
+ {"expect": {"dialog": null, "sketch_curves": {"own": 2}, "errors": 0}},
+ {"expect": {"query": {"command": "sketch.inspect", "path": ["curves", 1, "periodic"], "value": true}}},
+ {"call": "engine.execute", "params": {"command": "edit.undo", "params": {}}},
+ {"expect": {"sketch_curves": {"own": 1}, "errors": 0}},
+ {"call": "engine.execute", "params": {"command": "edit.redo", "params": {}}},
+ {"expect": {"sketch_curves": {"own": 2}, "errors": 0}}
+]