feat: preserve modeling intent across sketches, assemblies, and fabrication

Persist typed sketch dimensions and bounded ordered configuration rules in format 8. Use exact occurrence paths and source identity for editing, movement, appearance, and mass. Preserve sheet material regions through cutting, folding, and exact DXF export; retain nested cavities and material islands.

Validation: cargo xtask ci passed, including 368 engine tests, 178 UI tests, 68 UI scenarios, 136 kernel tests, and the 10-crate wasm check. Actual preview workflows and legacy format-7 origin-corner reopening were inspected.

crates/engine/src/cmd/sheet.rs

@@ -274,11 +274,17 @@ fn sheet_of(s: &Session, p: &Value, cmd: &str) -> Result<SheetBody> {
 }
 
 fn flat_json(sh: &SheetBody) -> Result<Value> {
-    let (w, h) = sh.flat_size()?;
-    let loops = sh.outline()?;
+    let regions = sh.flat_regions()?;
+    let loops: Vec<Vec<Vec2>> = regions.iter().map(|r| r.outer.polyline(1e-3)).collect();
+    let (mut lo, mut hi) = (Vec2::new(f64::MAX, f64::MAX), Vec2::new(f64::MIN, f64::MIN));
+    for p in loops.iter().flatten() {
+        lo = Vec2::new(lo.x.min(p.x), lo.y.min(p.y));
+        hi = Vec2::new(hi.x.max(p.x), hi.y.max(p.y));
+    }
+    let (w, h) = (hi.x - lo.x, hi.y - lo.y);
     let pt = |v: &Vec2| json!([v.x, v.y]);
     let outline: Vec<Value> = loops.iter().map(|l| Value::Array(l.iter().map(pt).collect())).collect();
-    let holes: Vec<Value> = sh.holes.iter().map(|h| Value::Array(h.outer.polyline(1e-3).iter().map(pt).collect())).collect();
+    let holes: Vec<Value> = regions.iter().flat_map(|r| &r.holes).map(|h| Value::Array(h.polyline(1e-3).iter().map(pt).collect())).collect();
     let bends: Vec<Value> = sh
         .bends()
         .iter()
@@ -331,16 +337,23 @@ pub fn flat_dxf(sh: &SheetBody) -> Result<String> {
         w(11, &f(b.x));
         w(21, &f(b.y));
     };
-    for l in sh.outline()? {
-        let m = l.len();
-        for i in 0..m {
-            if let (Some(a), Some(b)) = (l.get(i), l.get((i + 1) % m)) {
-                line(&mut w, "OUTLINE", *a, *b);
+    let regions = sh.flat_regions()?;
+    for h in regions.iter().flat_map(|r| std::iter::once(&r.outer).chain(&r.holes)) {
+        // A circle is often represented by two semicircles. Keep the whole analytic
+        // boundary as one CIRCLE rather than approximating outer boundaries with chords.
+        if let Some(solvecraft_geom::Seg2::Arc { center, radius, sweep, .. }) = h.segs.first() {
+            let total: f64 = h.segs.iter().filter_map(|s| if let solvecraft_geom::Seg2::Arc { sweep, .. } = s { Some(*sweep) } else { None }).sum();
+            let circular = h.segs.iter().all(|s| matches!(s, solvecraft_geom::Seg2::Arc { center: c, radius: r, sweep: a, .. } if c.dist(*center) < 1e-9 && (*r-*radius).abs() < 1e-9 && a.signum() == sweep.signum()));
+            if circular && (total.abs() - std::f64::consts::TAU).abs() < 1e-9 {
+                w(0, "CIRCLE");
+                w(8, "OUTLINE");
+                w(10, &f(center.x));
+                w(20, &f(center.y));
+                w(40, &f(*radius));
+                continue;
             }
         }
-    }
-    for h in &sh.holes {
-        for seg in &h.outer.segs {
+        for seg in &h.segs {
             match *seg {
                 solvecraft_geom::Seg2::Arc { center, radius, start, sweep } => {
                     if (sweep.abs() - std::f64::consts::TAU).abs() < 1e-9 {