feat(underlay): render PDF underlays end-to-end

PDF underlays only drew their clip outline / insertion cross; the page
itself never appeared. The chain now runs: the definition's file path
resolves with the raster-image fallbacks (as stored, relative to the
drawing, basename next to the drawing — attach-time relative paths go
stale), the page rasterises through the system pdftoppm (poppler) at
150 dpi — no new crate; missing tool degrades to the outline — cached
per (path, page) including negative results, and the pixels feed the
existing image pipeline as an ImageModel: 1 underlay unit = 1 PDF inch
(px/dpi × entity scale), rotated about the insertion, fade as opacity,
the clip polygon (non-inverted) as the textured region. Draw-order,
layer visibility and per-viewport freezes ride the generic image path;
in-place edits rebuild through the shared image seed.

When the page renders, the wire path draws its frame with an interior
pick surface instead of the lone cross, so selection covers the visible
page. wasm keeps the outline placeholder (no external process).

Co-Authored-By: Claude Fable 5 <[email protected]>

src/entities/underlay.rs

@@ -37,6 +37,38 @@ fn cross_wire(origin: [f64; 3], size: f64) -> Vec<[f64; 3]> {
 
 // ── TruckConvertible ──────────────────────────────────────────────────────────
 
+/// The underlay's page rectangle in world space (CCW from the insertion),
+/// when its definition resolves and the page rasterises: page inches × the
+/// entity scale, rotated about the insertion. `None` keeps the cross
+/// placeholder.
+fn page_quad(
+    u: &Underlay,
+    document: &acadrust::CadDocument,
+) -> Option<[[f64; 3]; 4]> {
+    let def = match document.objects.get(&u.definition_handle) {
+        Some(acadrust::objects::ObjectType::UnderlayDefinition(d)) => d,
+        _ => return None,
+    };
+    let page = if def.page_name.trim().is_empty() {
+        "1"
+    } else {
+        def.page_name.trim()
+    };
+    let raster = crate::scene::model::pdf_raster::rasterize_page(&def.file_path, page)?;
+    let w = raster.width as f64 / raster.dpi as f64 * u.x_scale;
+    let h = raster.height as f64 / raster.dpi as f64 * u.y_scale;
+    let (c, s) = (u.rotation.cos(), u.rotation.sin());
+    let o = &u.insertion_point;
+    let ux = (c * w, s * w);
+    let vx = (-s * h, c * h);
+    Some([
+        [o.x, o.y, o.z],
+        [o.x + ux.0, o.y + ux.1, o.z],
+        [o.x + ux.0 + vx.0, o.y + ux.1 + vx.1, o.z],
+        [o.x + vx.0, o.y + vx.1, o.z],
+    ])
+}
+
 impl TruckConvertible for Underlay {
     fn to_truck(&self, _document: &acadrust::CadDocument) -> Option<TruckEntity> {
         let origin = v3(&self.insertion_point);
@@ -64,6 +96,19 @@ impl TruckConvertible for Underlay {
                 key_vertices: key,
                 fill_tris: vec![],
             })
+        } else if let Some(q) = page_quad(self, _document) {
+            // Rendered page: draw its frame so selection/pick cover the
+            // visible extent instead of a lone cross under the raster.
+            let pts = vec![q[0], q[1], q[2], q[3], q[0]];
+            let pick_tris = crate::entities::mesh::triangulate_planar(&q.to_vec());
+            Some(TruckEntity {
+                pick_tris,
+                object: TruckObject::Lines(pts),
+                snap_pts: vec![(glam::DVec3::new(self.insertion_point.x, self.insertion_point.y, self.insertion_point.z), SnapHint::Node)],
+                tangent_geoms: vec![],
+                key_vertices: q.to_vec(),
+                fill_tris: vec![],
+            })
         } else {
             // No clip boundary: draw a cross at insertion point.
             let pts = cross_wire(origin, 1.0);