feat(sweep): associative swept surfaces in the drawing's network

Swept surfaces made with SURFACEASSOCIATIVITY on keep their profile and
path and are written as a swept-surface action of the drawing's
associative network, reading the profile and path and their scale and
twist expressions. They follow edits of their sources.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/settings.rs

@@ -412,6 +412,8 @@ pub struct UserSettings {
     pub attdef_on_screen: bool,
     /// FIELDDISPLAY: fields show on a gray background.
     pub field_display: bool,
+    /// SURFACEASSOCIATIVITY: new surfaces stay associative to their sources.
+    pub surface_associativity: bool,
     /// Nested-copy symbol handling: false inserts, true binds.
     pub ncopy_bind: bool,
     /// Last Options page; unknown saved names fall back without rejecting the config.
@@ -823,6 +825,7 @@ impl Default for UserSettings {
             image_frame: 1,
             attdef_on_screen: true,
             field_display: true,
+            surface_associativity: true,
             ncopy_bind: false,
             cursor_type: CursorType::Crosshair,
             crosshair_color: None,