Local appearance and material libraries (#863)

Adds local scalar appearance and material libraries: a catalog in the io crate with safe writes, engine commands to list and apply library entries, and a library dialog in the UI, with docs.

crates/ui-egui/src/dialogs_library_tests.rs

@@ -0,0 +1,41 @@
+use super::*;
+use crate::{dialogs::Kind, scenario::Harness};
+use solidcraft_engine::{Sel, Session};
+
+#[test]
+fn catalog_clicked_load_apply_export_and_ok_keep_one_design_undo_step() {
+    let mut h = Harness::new();
+    h.call("ui.resize", json!({"width":1600,"height":1400}));
+    let path = h.dir.join("source-library.json").to_string_lossy().into_owned();
+    let mut source = Session::default();
+    source.execute("solid.box", &json!({"length":10,"width":10,"height":10})).unwrap();
+    source.execute("appearance.assign", &json!({"bodies":["Body1"],"appearance":"Warm oak colour","color":[180,120,70],"opacity":0.8})).unwrap();
+    source.execute("library.export", &json!({"path":path})).unwrap();
+    h.app.run("solid.box", json!({"length":10,"width":10,"height":10})).unwrap();
+    h.app.run("appearance.assign", json!({"bodies":["Body1"],"color":[20,20,20]})).unwrap();
+    h.app.session.mark_saved();
+    h.app.session.selection = vec![Sel::Body { name: "Body1".into() }];
+    h.app.start("appearance.assign");
+    if let Some(dialog) = &mut h.app.dialog
+        && let Kind::Appearance(state) = &mut dialog.kind
+    {
+        state.local.path = path;
+    }
+    h.frames(3);
+    let undo = h.app.session.undo.len();
+    h.step(&json!({"widget":"Load library"})).unwrap();
+    h.step(&json!({"click":{"handle":"library:entry:0"}})).unwrap();
+    h.step(&json!({"widget":"Apply library appearance"})).unwrap();
+    assert_eq!(h.app.session.doc.appearances.bodies["Body1"].name, "Warm oak colour");
+    assert_eq!(h.app.session.undo.len(), undo + 1);
+    let revision = h.app.session.revision;
+    h.step(&json!({"widget":"Export design library..."})).unwrap();
+    let exported = h.dir.join("my-library.solidcraft-library.json");
+    let catalog = Catalog::read(&exported.to_string_lossy()).unwrap();
+    assert_eq!(catalog.entries.len(), 1);
+    assert_eq!(catalog.entries[0].definition.name(), "Warm oak colour");
+    assert_eq!(h.app.session.revision, revision);
+    h.step(&json!({"widget":"OK"})).unwrap();
+    assert!(h.app.dialog.is_none());
+    assert_eq!(h.app.session.undo.len(), undo + 1);
+}