oneOf detection too broad: only a simple const-oneOf is choices; a deep oneOf is a variant/section selector

Discovered during 4j1 demo work (App_14 oneOf multiselect + App_01 country select).

BUG: the parser treats ANY oneOf as a finite set of choices:

• packages/core/src/parser/arrayNode.ts isPrimitiveArraySchema(): if (itemsSchema.enum || itemsSchema.oneOf) return true -> the whole array becomes a multiselect FieldNode.
• packages/core/src/parser/fieldNode.ts createFieldNode(): hasOneOf -> isSelect; buildSelectOptions() maps oneOf branches, filtering ('const' in item).

But oneOf is also (more commonly) a STRUCTURAL union: branches that are object subschemas (discriminated union) meant to swap out whole form sections, e.g. items.oneOf = [ {type:object, properties:{...A}}, {type:object, properties:{...B}} ]. Those branches have no const, so today they are FILTERED OUT -> an empty-options multiselect. The array never becomes an add/remove ArrayNode either, so the entire variant structure is silently dropped.

Only a SIMPLE oneOf should be treated as choices: every branch is a scalar constant (has const, primitive value), i.e. an enum-with-labels exactly like App_01 country ([{const:'US',title:'United States'}, ...]).

FIX (parser-side, front-end concern): gate the "oneOf -> choices" detection on ALL branches being simple const entries. If any branch is object/array/$ref/typed-without-const, it is NOT choices — treat it as a (currently unsupported) union/variant selector, a separate feature (discriminated-union / section-swap). At minimum, do not mis-collapse it into an empty multiselect.

Lower priority (per Tim). No consumer hits it until deep-oneOf schemas are used.

Relationships

• Discovered from: jsonschema-form-4j1 (not migrated)

_{bd: jsonschema-form-aml}

Start in packages/core/src/parser/arrayNode.ts and packages/core/src/parser/fieldNode.ts, tracing isPrimitiveArraySchema(), createFieldNode(), and buildSelectOptions(). Check the App_01 simple const-oneOf and the App_14 deep object-oneOf cases; done means scalar const branches remain choices while structural branches are not collapsed into an empty multiselect.