feat: edge flanges with their own angle and height per edge (#447)

One edge flange feature gave all its flanges one height and one angle. A
part whose flanges lean differently took one feature per angle, and
flanges of separate features don't close toward each other, so its
corners stayed open.

- sheet.flange (edge): an entry of `edges` is a point [x, y, z] as before,
  or {point, angle?, height?} whose angle and height (expressions, checked
  like the feature's) replace the feature's for that edge. Bad input is
  refused with the edge named. Before, the object form was read for its
  point and the rest was ignored without a word.
- The feature stores its edges as FlangeEdge. An edge without values of
  its own is written as the plain point it always was, so designs without
  them are read and written as before; an unknown key in an edge is
  refused, not dropped.
- The edges' values are feature inputs of their own (Edge angle, Edge
  height, after the feature's), so parameters, renames and
  parameters.change reach them. timeline.edit names the edges again when
  it changes them, so each value stays with its edge.
- SheetBody::add_flanges_each trims every edge by its own setback and
  adds the flanges. The corners between them close as between equal
  flanges: the closing already works from the two flanges it joins (each
  one's fold and leg), so the flat pattern follows by hand from the
  angles, the heights and the rule (the tests derive it).
- The Flange dialog shows a row per picked edge with an angle and a
  height box (empty: the dialog's value) once more than one edge is
  picked, and brings them back on Edit Feature.

Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/book/src/guide/sheet-metal.md

@@ -24,6 +24,24 @@ opposite ways, ends that would overlap in the flat pattern) stays open, and the
 reports a warning saying where. A flange added by a later feature leaves the corner as it was:
 the earlier flange keeps its length and the new one ends at its bend.
 
+The flanges of one edge flange feature can each have an angle and a height of their own. An
+entry of `edges` is a point `[x, y, z]`, which takes the feature's `height` and `angle`, or an
+object `{"point": [x, y, z], "angle": "105 deg", "height": "30 mm"}` whose `angle` and `height`
+(either one, or both) replace them for that edge. They are expressions like the feature's own,
+so parameters work, and each is a feature input of its own (`Edge angle`, `Edge height`) after
+`Height` and `Angle`. The corners between such flanges close like those between equal ones:
+the ends run toward each other the rule's gap apart, either side of the plane halfway between
+the two flanges, and a taller flange's end keeps that line up to its own tip. This is how to
+close a corner between flanges at different angles, because flanges of separate features
+don't close toward each other. In the Flange dialog, picking more than one edge shows a row
+per edge with an angle and a height box; an empty box takes the value above.
+
+```json
+{"command":"sheet.flange","params":{"edges":[[60,0,2],{"point":[120,45,2],"angle":"90 deg"},
+ {"point":[60,90,2],"angle":"105 deg","height":"30 mm"},{"point":[0,45,2],"angle":"120 deg","height":"25 mm"}],
+ "height":38.6,"angle":"75 deg"}}
+```
+
 For a shop drawing or spreadsheet, `sheet.export_bend_table` returns CSV by default; choose
 `format: "json"` for structured data. It includes the sheet rule, thickness, bend angles,
 radii, allowances and flat centre-line coordinates. Lengths use millimetres and angles use