fix: edge flanges trim the base along their own edge and relieve bends that end beside it (#634)

An edge flange whose edge ends at an inside corner of the base now trims only the strip under its own edge, so legs and stems past that corner stay. Where the bend ends beside more of the base, the rule's bend relief (relief_width, relief_depth) is cut into the base, or a note says why it was left out.

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.
 
+An edge flange takes its setback off its own edge only. Where that edge ends at an inside
+corner of the base (the base runs on past it, like the leg of an L), the rest of the base
+stays, and the rule's **bend relief** (`relief_width` and `relief_depth`; Relief Width and
+Relief Depth in Sheet Metal Rules) is cut into it beside the bend's end: a slot the relief
+width wide, from the relief depth before the bend line to the relief width past the flange's
+far edge in the flat pattern. The bend and the flange are then free of the base, folded and
+flat. A relief that can't be cut (a width under 0.01 mm, one as wide as the base beside the
+bend, or a slot that would cut the base apart) is left out, and the flange feature reports a
+warning saying where.
+
 Where two bends of one edge flange feature meet at a corner of the base, the rule's **corner
 relief shape** (`corner_relief_shape`; Corner Relief Shape in Sheet Metal Rules) says what is
 cut there. `notch`, the default, cuts nothing more: the bends end square at the corner. `round`