fix(pdf): per-piece snap bounds, linear image layering, snap op checks

Review of #1484:
- The underlay's snap geometry became one hidden wire per segment, but
  every piece took the whole page's bounding box, so each cursor move
  visited every piece. Each piece now gets its own box.
- Assigning unlayered images to the last layer compared Debug strings of
  every layered path and text per drawn item (quadratic, formatted anew
  each time); the owners are keyed once in a map.
- The `snap` automation op used (0, 0, 0) for a missing point; it now
  answers invalid_point. It also left the live snapper's base point set
  to the query's; the base point is restored.

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

src/app/control/mod.rs

@@ -708,13 +708,12 @@ impl OpenCADStudio {
                 "measure" => self.control_measure(&req),
                 // The object snap a cursor over `point` (world) would get;
                 // `from` is the base for perpendicular and tangent.
-                "snap" => {
+                "snap" => 'snap: {
                     let p = &req["point"];
-                    let world = glam::DVec3::new(
-                        p[0].as_f64().unwrap_or(0.0),
-                        p[1].as_f64().unwrap_or(0.0),
-                        p[2].as_f64().unwrap_or(0.0),
-                    );
+                    let (Some(x), Some(y)) = (p[0].as_f64(), p[1].as_f64()) else {
+                        break 'snap failure("invalid_point", "point must be [x, y] or [x, y, z]");
+                    };
+                    let world = glam::DVec3::new(x, y, p[2].as_f64().unwrap_or(0.0));
                     let from = req["from"].as_array().map(|f| {
                         glam::DVec3::new(
                             f.first().and_then(|v| v.as_f64()).unwrap_or(0.0),