Thin Extrude: walls of a set thickness along sketch curves (#721) (#991)

Thin Extrude builds walls of a set thickness along sketch curves, open chains and closed loops, with the wall centred, inside, outside or on both sides at two thicknesses. Saved under format 20 (thin on an Extrude); an extrude without thin saves as before. Closes #721.

docs/book/src/guide/features.md

@@ -7,7 +7,7 @@ it.
 
 | Panel | Features |
 |---|---|
-| Create | extrude (one side, two sides, symmetric, taper, start offset, to a distance or through all; new body, join, cut, intersect), revolve, sweep, loft, box, cylinder, sphere, torus, coil, pipe, rib, web, emboss, hole (simple, counterbore, countersink, tapped; on faces or at sketch points), thread, rectangular/circular/path patterns, mirror |
+| Create | extrude (one side, two sides, symmetric, taper, start offset, to a distance or through all; new body, join, cut, intersect; thin walls along curves), revolve, sweep, loft, box, cylinder, sphere, torus, coil, pipe, rib, web, emboss, hole (simple, counterbore, countersink, tapped; on faces or at sketch points), thread, rectangular/circular/path patterns, mirror |
 | Modify | press pull, fillet, chamfer, shell, draft, scale, combine, split body, offset face, replace face, move/copy, align, remove |
 | Construct | offset plane, plane at angle, plane along path |
 | Inspect | measure, section analysis, interference, curvature comb and map, zebra, draft, minimum radius, isocurves, centre of mass |
@@ -23,6 +23,19 @@ curves move in or out together, and a flat tip shorter than the taper closes to
 taper steeper than the profile's details (a stroke, a curve or a gap narrower than it) is
 refused with that reason; a smaller taper or distance builds.
 
+**Thin Extrude** (`thin: true` on `solid.extrude`, the Thin Extrude box in its dialog) builds a
+wall of constant thickness along sketch curves instead of filling profiles: open chains (a single
+wall) and closed loops (a ring), lines, arcs, circles, ellipses and splines. In the dialog, a
+picked profile stands for the curves around it (its outline and its holes). The wall lies on the curves' centre, inside,
+outside, or on both sides with two thicknesses (`side`: `center`, `inside`, `outside`,
+`two_sides` with `thickness2`). A closed loop's inside is the area it encloses; an open chain's
+is the side it bends toward (the inside of a U), and a straight line's is the left of its first
+curve as drawn. Curves that meet end to end are one chain whose corners are mitred; walls that
+cross or meet (a ring and a cross, a honeycomb) become one piece. Every extent, operation and target of an extrude applies, and a taper
+for walls of straight lines or of whole circles. A wall thicker than the
+radius of a curve it follows on the inside is refused. Spline walls follow the exact offset
+within 0.0001 mm.
+
 Editing an extrusion retains its signed distances, From/To Object inputs, taper expressions
 and explicit body targets. Revolve edits retain world, sketch and explicit-line axes, both
 angle expressions and To Object targets. An unchanged **All profiles** selection continues
@@ -124,7 +137,7 @@ still be added through `set`. The fields of the common features:
 
 | Feature | Fields |
 |---|---|
-| Extrude | `extent`: {`distance`, `direction`: positive\|negative\|symmetric, `distance2`, `start_offset`, `through_all`, `taper`, `to`, `to_offset`, `from`}; `operation`: new_body\|join\|cut\|intersect; `targets`; `profiles`; `sketch` |
+| Extrude | `extent`: {`distance`, `direction`: positive\|negative\|symmetric, `distance2`, `start_offset`, `through_all`, `taper`, `to`, `to_offset`, `from`}; `operation`: new_body\|join\|cut\|intersect; `targets`; `profiles`; `sketch`; `thin`: {`curves`, `thickness`, `side`: center\|inside\|outside\|two_sides, `thickness2`} |
 | Revolve | `angle`, `angle2`, `symmetric`, `axis`, `to`, `operation`, `targets`, `profiles`, `sketch` |
 | Fillet | `radius`, `edges` (a point on each edge), `body`, `style` |
 | Chamfer | `distance`, `distance2` or `angle`, `flip`, `edges`, `body` |

docs/book/src/guide/files.md

@@ -73,7 +73,7 @@ is available. Missing or incompatible pictures use the usual thumbnail generatio
 The default `cache: false` omits the portable thumbnail too.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
-Current designs use format 19, including editable associative offset-distance dimensions, the style of sketch text (bold, italic, alignment, spacing and the curve it follows), face anchors on a piece of a face by its own name, sheet metal rims and bend ends, bend edges for Convert to Sheet Metal, the tear corner relief of sheet metal rules, the names of projected edges, physical materials defined in the design, the font of sketch text, sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis
+Current designs use format 20, including thin extrudes, editable associative offset-distance dimensions, the style of sketch text (bold, italic, alignment, spacing and the curve it follows), face anchors on a piece of a face by its own name, sheet metal rims and bend ends, bend edges for Convert to Sheet Metal, the tear corner relief of sheet metal rules, the names of projected edges, physical materials defined in the design, the font of sketch text, sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis
 links, persistent
 names from exact face history, exact planar shell-face anchors, persistent analytic offset boundary roles and
 association profile references, durable sketch face anchors, periodic fit splines and versioned