UI: a reference field is always a drop-down; with nothing to pick it offers Create …, and one candidate starts picked (#215)

A reference field (a material, set, step…) with no items was a free-text box; it is now a greyed
drop-down ("No materials yet") with a button that opens the command making one (the
collection's requirement provider). That form comes back to this one on OK, with the new item
picked, or as it was on Cancel. A required reference with one candidate, or with the browser's
selection of its kind, starts picked when the form opens, so the fields and the JSON view agree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/browser.rs

@@ -9,6 +9,7 @@
 //! ATTRIBUTION.md.
 
 use egui::{Color32, FontId, Pos2, Rect, RichText, Sense, Stroke, Vec2, pos2, vec2};
+use fieldcraft_engine::schema::Collection;
 use fieldcraft_model::Model;
 use serde::Serialize;
 use serde_json::{Value, json};
@@ -39,6 +40,22 @@ pub enum Def {
 }
 
 impl Def {
+    /// The reference fields that name one of these.
+    pub fn collection(self) -> Collection {
+        match self {
+            Def::Set => Collection::Set,
+            Def::Material => Collection::Material,
+            Def::Section => Collection::Section,
+            Def::Step => Collection::Step,
+            Def::BoundaryCondition => Collection::BoundaryCondition,
+            Def::Load => Collection::Load,
+            Def::ReferencePoint => Collection::ReferencePoint,
+            Def::Coupling => Collection::Coupling,
+            Def::Contact => Collection::Contact,
+            Def::FluidDomain => Collection::FluidDomain,
+        }
+    }
+
     /// The commands' stem: `model.<stem>.create|edit|delete`.
     pub fn stem(self) -> &'static str {
         match self {