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

docs/control-protocol.md

@@ -51,7 +51,7 @@ commands (files, undo, `model.info`) leave the ribbon where it is.
 | `ui.select` (viewport) | any of `filter`, `box_mode`, `mode`, `clear`, `at`, `box`, `geometry`, `nodes`, `elements`, `faces` (and no `kind`) | changes the viewport's selection (see [Selection](#selection)); returns `ui.selection`'s reply |
 | `ui.selection` | — | the viewport's selection: `filter`, `box_mode` (box selection on), `count`, `capped`, `summary`, `items`, `params` (the selection as a set's members), `hover` (what a click would pick under the pointer) |
 | `ui.prompt` | `answer`: for `unsaved` `save`, `discard` or `cancel`; for `design` `new` (a new model with the design), `keep` (the design over the current model) or `cancel`; for `recover` `recover` (with `id` from `file.recovery.list`) or `close`; for `samples` `open` (with `name`) or `close` | answers the window's question (`ui.inspect`'s `prompt`) |
-| `ui.dialog` | `params?` (JSON object, fills the form), `use_selection?`, `run?`, `cancel?` | drives the open command form, in that order; `use_selection` is the form's Target row (see [Selection](#selection)); `run` returns the command's result and closes the form on success |
+| `ui.dialog` | `params?` (JSON object, fills the form), `use_selection?`, `run?`, `cancel?` | drives the open command form, in that order; `use_selection` is the form's Target row (see [Selection](#selection)); `run` returns the command's result and closes the form on success; a form opened by another form's **Create …** button (a reference field with nothing to pick) goes back to that form on `run` (with the new item picked) or `cancel` |
 | `ui.click` | `x`, `y` (screen points), `button?`: `left`/`right`/`middle`, `shift?`, `ctrl?`, `alt?`, `double?` | real pointer input; in the viewport a click selects what is under it (Shift adds, Ctrl/Cmd adds or removes, a click on nothing clears the selection), and a click on a results plot also probes the nearest node of the face under it (`solve.probe`; `ui.inspect` shows the values) |
 | `ui.move` | `x`, `y` | |
 | `ui.drag` | `x0`, `y0`, `x1`, `y1`, `button?`, `shift?`, `ctrl?`, `steps?` (1–200) | press, move, release: in the viewport a left drag orbits (with box selection on, it selects the visible nodes, element faces or elements in the box), a right or middle drag (or Shift + left) pans |