feat: pick sketch profiles by their bounding curve ids so features keep the same region when parameters move the geometry, matching curves cut into pieces, refusing ambiguous or missing picks instead of taking another region, and storing point picks by curves

docs/book/src/guide/sketching.md

@@ -32,3 +32,12 @@ constraint while you drag.
 
 Closed loops become profiles automatically, including regions split by crossing lines and
 islands inside outlines. Extrude, revolve, sweep and loft pick profiles.
+
+A feature remembers a picked profile by the curves around it, not by where it was clicked:
+`"profiles": [{"curves": ["l1", "l2", "a1", "l3"]}]` (or `[{"curves": ["c1"]}]` for a circle)
+picks the region whose outer loop is made of exactly those curves, or the pieces of them that
+other curves cut off. It is found again on every recompute, so the feature keeps the same region
+when a parameter moves the geometry. Islands inside the region are its holes. When no region, or
+more than one (both halves of a circle cut by a line), is bounded by exactly those curves, the
+feature reports an error rather than taking another region. Profiles picked by index or by point
+(`[{"point": [x, y]}]`) are stored the same way when the feature is made.