fix: a projected edge stays the edge that was picked when its body changes (#730)

A sketch's link to a projected edge now keeps the edge's persistent name and finds it by that name when the body changes, instead of taking whichever edge ends up nearest its point.

docs/book/src/dev/naming.md

@@ -80,6 +80,14 @@ longer lies on the face says so: `face ... was found by geometry, not exact hist
 An edge name is found as the edge between the two faces it names, each found as above: when
 exactly one edge runs between them it is that edge, even after a split of one of its faces.
 
+A sketch's link to a projected edge (`sketch.project {edge: [x, y, z]}`, and Intersect and
+Include 3D Geometry of an edge) keeps the edge's name beside its point. The link is that edge
+wherever a change takes it; of an edge that was split it is the piece nearest the link's point,
+and the sketch says so. When no edge has the name any more the link is lost and keeps its last
+geometry, as it does for a face: it does not take whichever edge is nearest now. A link saved
+without a name, by a build that kept only the point, is found by the edge nearest its point, as
+it always was, and takes that edge's name when its sketch is next edited.
+
 A hole placed by a position on a face is the exception to "the point nearest the reference's
 point": its position is a place, not a pick. The hole follows its face along its own axis, so it
 moves with the face when the face moves, and when the face no longer reaches the position it stays

docs/book/src/guide/files.md

@@ -58,7 +58,7 @@ 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 17, including 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, 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 17, including 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 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

docs/book/src/guide/sketching.md

@@ -59,6 +59,8 @@ design unchanged. Projected geometry remains protected from ordinary dragging.
 
 **Project** (P) and **Intersect** bring model edges into the sketch as linked geometry that
 follows later changes. Sketching on a face projects its edges automatically.
+A projected edge stays the edge that was picked, wherever a later change moves it; when that
+edge no longer exists, the projection keeps its last shape and the sketch says it was lost.
 For Project, select visible source curves or points from another sketch, model edges,
 faces, vertices or bodies, or axes and planes, then press P. The entire supported selection
 is projected in one undo step. A rejected projection keeps the selection for correction;