feat: close sheet flange corners at any angle (#406)

Edge flanges meeting at a corner of the base were closed only at exactly
90°; at any other angle they stayed open (a V at 72°) and the rule's gap
was not used.

The flanges of one feature now close at any convex base corner and any
bend angle: each panel end is cut square to the sheet on a line in the
flat (slanted off 90°, stored as ext plus a new per-end slope) so the
edges nearest each other, on the inner faces, run parallel the rule's gap
apart about the plane bisecting the inner faces. The construction reads
the two flanges' own angles, so it holds for unequal ones as well. At 90°
it is the extension the old code gave (reference case 69-sm-tray is
unchanged). A corner that can't close (a gap under 0.01 mm, flanges
bending opposite ways, ends that would overlap in the flat pattern) stays
open, and the flange feature reports a warning that says where and why.

A flange of a later feature leaves the corner as it was (reference case
75-sm-bracket-holes: the earlier flange keeps its length).

Tested at 72° and 90° against a hand derivation, on a trapezoid, and
every 5° from 45° to 135° on a rectangle and a trapezoid (gap exact, one
flat region without hairline edges, one valid shell holding the flat
material).

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

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

@@ -14,6 +14,16 @@ The SHEET METAL tab builds parts from a sheet of constant thickness under a **sh
 Bends follow the rule's K-factor, so flat lengths match what a press brake will make. Tests
 check flat lengths against hand-computed bend allowances.
 
+Where the edge flanges of one feature meet at a corner of the base, their panels are closed
+toward each other at any bend angle: each end is cut square to the sheet, on a line in the flat
+chosen so that the edges nearest each other (on the inner faces) run parallel, the rule's
+**gap** apart, either side of the plane halfway between the flanges. Flanges at 72° get slanted
+ends in the flat; corners that aren't square close the same way, and at 90° it is the corner
+the rule always gave. A corner that can't be closed (a gap under 0.01 mm, flanges bending
+opposite ways, ends that would overlap in the flat pattern) stays open, and the flange feature
+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.
+
 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