Fix #7525: Extrude & Bevel cuts sharp corners square instead of spiking

An inset ring limited a corner's miter by moving the corner four times
the bevel width along its bisector. At a corner sharper than about 29°
that the bevel opens (a convex corner with the bevel outside, a notch
with it inside), the bevel still ran out to a spike up to four times
the width and no longer lined up with the art. With the bevel outside,
the wall behind it spiked the same way.

Such a corner is now given twice in the outline before the solid is
built, and each copy moves along its own edge's normal, so the bevel
(and, with the bevel outside, the wall) cuts the corner square, as a
bevel join cuts a stroke. Corners within the limit keep their miters,
and corners where the inset edges meet keep the four-times limit.

A cap's outline skips the straight segment of no length between the
two copies where they do not move apart.

Docs: docs/three-d.md, docs/target-app-parity.md.

docs/target-app-parity.md

@@ -1266,6 +1266,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - When the fill and strokes are at full opacity with Normal blending, under a stroke that leaves parts of the fill's edge uncovered (dashed, with a width profile or a brush, on an open subpath, or in a paint that may show through), Extrude & Bevel (both kinds) and Inflate cut each stroke face that passes through a face of the fill's solid, or of another of the shape's strokes, where their depths meet, and draw the faces of the shape's fill and strokes that overlap in the order of their depths where they overlap, instead of by the depths of their centers. Inflate orders a plain stroke the same way, since its fill can rise in front of the stroke. The fill no longer paints over a nearer dash, and a dash's end no longer covers the fill wall in front of it; a fill wall in a gap between dashes stays visible. The same order applies inside a group member's own opacity mask or blending mode, to a gradient with transparent stops under a colored light with graphics on its caps, and to Inflate in gradients under a colored light. Tests: `solids::tests::fill_walls_and_strokes_that_leave_gaps_draw_in_the_order_of_their_depths_where_they_overlap`, `solids::tests::local_order_puts_overlapping_fill_walls_and_stroke_faces_in_depth_order`, `solids::tests::a_stroke_face_through_a_fill_wall_is_cut_where_their_depths_meet`, `solids::tests::a_masked_members_fill_and_stroke_faces_go_in_one_order`, `solids::tests::a_colored_inflate_in_gradients_draws_its_faces_in_one_order`, `solids::tests::a_depth_cycle_keeps_the_cap_last`, `solids::graphics::tests::a_transparent_gradient_with_a_graphic_keeps_the_shared_face_order`.
   - Under a colored light, an Inflate fill and strokes in solid colors, opaque with Normal blending, also draw their faces in that one order, inside a group member's own opacity mask too, instead of each solid as one composite with the stroke's faces over every fill face. Each face keeps the composite's paint, light ramps and seam width; a lone solid, the solids of an object with a translucent or blended fill or stroke, and such solids holding more than 30,720 faces together (VectorCraft's own limit) are still drawn as one composite each. Tests: `solids::tests::colored_solid_fills_and_strokes_share_face_depth_order`, `solids::tests::a_colored_solid_face_on_its_own_keeps_the_composites_colors`, `solids::tests::colored_solid_inflate_batches_channels_and_clips_once_per_surface`, `solids::tests::colored_solids_past_the_face_limit_draw_as_composites`.
   - Without a twist, a tapered Extrude & Bevel wall (and each bevel band) is one flat face; only a twist cuts walls into slices, so no seam between slices paints a light tick or a light line on a tapered, untwisted wall (#7019). Test: `solid::tests::a_tapered_wall_is_one_flat_face_and_a_twisted_one_is_sliced`.
+  - Extrude & Bevel cuts a corner square where the bevel opens it and its miter would reach past four times the width (a sharp point with the bevel outside, a narrow notch with it inside), so the bevel (and, with the bevel outside, the walls) no longer runs out to a spike that does not line up with the art (#7525). Tests: `solid::tests::a_bevel_cuts_sharp_corners_square`, `solid::tests::a_cap_draws_no_empty_segment_at_a_corner_cut_square`.
   - Extrude & Bevel, Inflate and Rotate apply to type: it makes a solid of its glyph outlines, painted with its own fills and strokes or, when it has none, its characters' colour (the first run's), and stays type, so editing the text reshapes the solid (#4715).
   - Extrude & Bevel, Inflate and Rotate (Effect › 3D and Materials): live effects in the `three-d` crate with depth, twist, taper, solid or hollow caps, five original bevel shapes (width, height, repeat, spacing, inside, both sides), Inflate's volume, both sides, twist, taper and cap, Rotate's turned fills and strokes, Position presets, the rotation and light gizmos, SVG/PDF vector output and Expand Appearance; one 3D effect per object; covered by geometry, effect, canvas-versus-export, command and dialog tests ([three-d.md](three-d.md)).
   - Extrude & Bevel and Inflate build detailed shapes: each solid takes up to 65,536 outline points and 163,840 faces (VectorCraft's own limits, was 2,048 points and 32,768 faces), counted before any point is sampled; when the faces do not fit, every curve takes fewer pieces, down to 4. A closed path of 2,000 curves extrudes with a bevel and inflates, and paths of 5,000 and 10,000 curves build at the default settings (`crates/three-d/src/solid/tests_limits.rs`).

docs/three-d.md

@@ -85,8 +85,10 @@ object is where the art was.
 | Bevel Inside | The bevel carved into the shape (the front face shrinks) instead of added around it (the walls stand outside the art). |
 | Bevel both sides | The back is bevelled too. |
 
-Holes stay holes, with walls of their own. Bevels move each corner along its bisector (mitres are limited to four
-times the width), so a bevel wider than a narrow part of the shape folds over itself. Each facet has a seam ([Seams](#seams)).
+Holes stay holes, with walls of their own. Bevels move each corner along its bisector. A corner the bevel opens (a
+sharp point with the bevel outside, a narrow notch with it inside) whose miter would reach past four times the width is
+cut square across instead; at the other corners the miter is limited to four times the width. A bevel wider than a
+narrow part of the shape folds over itself. Each facet has a seam ([Seams](#seams)).
 
 ## Inflate