Web app: open and insert STEP files over frames, with progress and Cancel (#1376) (#1424)

* Web app: read a STEP file a piece per frame, with progress and Cancel (#1376)

The kernel's STEP reader walks the file first and converts its solids afterwards,
and StepReading exposes those stages: parse and walk, then convert a solid (or a
batch) per step, then finish into the shared import cache. The web app's file
load drives it from the frame loop for about 30 ms a frame, shows the same
progress card and Cancel as the desktop, and then runs the insert or open, which
finds the file already read. The desktop path is unchanged in behaviour.

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

* Web app: make the imported bodies' meshes a few per frame too (#1376)

After the read, the insert or open runs, and then the bodies' display meshes are
made a frame's worth at a time ("Preparing the view"), as the desktop worker
warms them, instead of all in the first frame that draws the design.

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

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/kernel/src/step_in/tests.rs

@@ -175,16 +175,19 @@ fn parallel_import_does_not_prepare_past_body_budget_with_duplicates_or_mapped_i
         "{HEAD}{}#2000=MANIFOLD_SOLID_BREP('Duplicate',#100);\n#2001=MANIFOLD_SOLID_BREP('First',#100);\n#2002=MANIFOLD_SOLID_BREP('Second',#100);\n#2003=MANIFOLD_SOLID_BREP('Beyond budget',#999999);\n#3000=MAPPED_ITEM('',#999998,#999997);\n#60=SHAPE_REPRESENTATION('',(#2000,#2000,#2001,#3000,#2002,#2003),$);\n{TAIL}",
         cube_entities()
     );
-    let ex = p21::parse(&text).unwrap();
+    let ex = std::sync::Arc::new(p21::parse(&text).unwrap());
     let seed =
         step_import_with_workers(&format!("{HEAD}{}#60=SHAPE_REPRESENTATION('',(#101),$);\n{TAIL}", cube_entities()), 1).unwrap().bodies.remove(0);
     for workers in [1, 8] {
-        let mut reader = Reader::new(&ex, workers);
+        let mut reader = Reader::new(ex.clone(), workers);
         reader.index();
         reader.bodies = vec![seed.clone(); MAX_BODIES - 2];
         reader.node_items.insert(2000);
         let mut out = Vec::new();
         reader.rep_bodies(60, &mt::Matrix4::from_scale(1.0), &[], "Budget", None, &mut out, 0);
+        assert!(reader.solids.is_empty(), "the walk only plans the solids");
+        while reader.convert_next(workers == 1) {}
+        let out = reader.remap(&out);
         assert_eq!(reader.bodies.len(), MAX_BODIES);
         assert_eq!(out, vec![MAX_BODIES - 2, MAX_BODIES - 1]);
         assert_eq!(reader.bodies.last().unwrap().name, "Second");
@@ -598,8 +601,8 @@ fn repeated_items_are_shared_and_one_use_items_are_not() {
     let a = box_solid(Vec3::ZERO, Vec3::new(10.0, 10.0, 10.0)).unwrap();
     let b = box_solid(Vec3::new(20.0, 0.0, 0.0), Vec3::new(30.0, 10.0, 10.0)).unwrap();
     let flat = step_export(&[&a, &b], "test").unwrap();
-    let ex = p21::parse(&flat).unwrap();
-    let mut rd = Reader::new(&ex, 1);
+    let ex = std::sync::Arc::new(p21::parse(&flat).unwrap());
+    let mut rd = Reader::new(ex.clone(), 1);
     rd.index();
     assert_eq!(rd.once.len(), 2, "each solid of a flat file is converted once");
     let imp = step_import(&flat).unwrap();
@@ -609,8 +612,8 @@ fn repeated_items_are_shared_and_one_use_items_are_not() {
     assert!(rel(vol, 2000.0) < 1e-6, "{vol}");
 
     let assembly = two_instance_assembly();
-    let ex = p21::parse(&assembly).unwrap();
-    let mut rd = Reader::new(&ex, 1);
+    let ex = std::sync::Arc::new(p21::parse(&assembly).unwrap());
+    let mut rd = Reader::new(ex.clone(), 1);
     rd.index();
     assert!(rd.once.is_empty(), "an assembly may convert an item again for the second instance");
     let imp = step_import(&assembly).unwrap();