fix(fields): group-labelled widget 的只读/零选项分支也消费 host label 的 IDREF (#3990)

.changeset/fields-readonly-group-label-idref-3990.md

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+---
+"@object-ui/fields": patch
+---
+
+Group-labelled field widgets now consume the host label's IDREF in their readonly and zero-option states, so the visible label names something there too
+
+A field declared `labelling: 'group'` (objectui#3961) is named by IDREF: the form
+renderer publishes its `<FormLabel>`'s own `id`, drops the inert `for`, and hands
+the widget `aria-labelledby` plus the host control `id`. All seven such widgets
+consumed that pair on their editable surface only. A field-level
+`readonly: true`, and an option list that resolved to zero offered options, take
+early returns that render before the container doing the consuming — so the label
+published an id that no element in the document referenced.
+
+Measured on the previous `main`, one field per row in a real form, counting the
+elements that reference the host label's published id:
+
+```
+                             consumers  byLabelText  named group
+multiselect  readonly+value      0           0           0
+multiselect  readonly+empty      0           0           0
+multiselect  zeroOptions         0           0           0
+multiselect  editable            1           1           1
+```
+
+All seven measured identically in every readonly state; each is 1 now. The
+user-visible effect is the one objectui#3961 fixed for editable fields: the
+visible label was the accessible name of NOTHING, so a readonly "Tags" or
+"Shipping Address" was announced as loose text next to unattributed values. It is
+not a regression of #3961 or objectui#3975 — before them these same states
+emitted a `for` pointing at an id no element carried, which named nothing either.
+The shape changed from a dangling `for` to an unconsumed IDREF; the defect did
+not.
+
+Each readonly surface now carries exactly two keys — the host `id` and
+`aria-labelledby` — plus the `role="group"` that makes them meaningful, in one
+shared spelling (`toHostGroupProps`). The narrow pair is deliberate: a readonly
+display has no focusable control, so `aria-describedby` / `aria-required` /
+`disabled` / the focus handlers have nothing to announce on, and `name` on a
+`div` is exactly the DOM leak objectui#3291 sweeps for.
+
+Two widgets answer with a different role than they do while editable, because
+they render a different surface: `radio` (editable `radiogroup`) shows the chosen
+option's label as text with no radios in it, and `file` (editable `button`, the
+dropzone) shows file names with no dropzone. `role="group"` is also what the
+shared "no options available" box now answers with for `checkboxes` / `radio` /
+`multiselect`; the single `select` is not group-labelled, keeps its working
+`for`, and that box emits nothing new for it.
+
+Standalone rendering is untouched. The grid's inline cell editor and bare SDUI
+nodes hand down neither key, so nothing is emitted and the markup stays
+byte-identical — including the `EmptyValue` placeholder, which keeps its own
+`aria-label` and gains no role. Hosted-and-empty, that placeholder is the whole
+readonly surface, so it carries the group props: its `aria-label` ("No value") is
+then outranked by `aria-labelledby` per accname, which is the intended outcome —
+on the `generic` role that span carries, an author name is prohibited and was
+never exposed, so the choice was between the field's name and no name.