[SDUI][P0] Form layout designer (slot overrides + visual editor)

[xuyushun441-sys]

Context

Create/Edit forms are still auto-synthesized from objectDef.fields (sections derived in RecordFormPage/ObjectForm). Unlike record detail pages, forms have no slot/override mechanism and no visual layout editor — field placement, grouping, columns, and visibility can't be authored.

Goal

Make form layout authorable: slot/section overrides in metadata + a visual editor, while keeping zero-config auto-synthesis as the default.

Scope

• Define a form layout schema (sections/columns/field placement/visibility) that overrides the auto-derived layout (similar to detail-page slots).
• A visual editor (extend PageDesigner or a dedicated FormLayoutDesigner) that edits it.
• Persist via the draft/publish seam (artifact type 'form' or under 'page').
• Auto-synthesis remains the fallback when no layout authored.

Acceptance

• A form can declare field grouping/columns/order/visibility via metadata
• Visual editor round-trips the layout (draft → publish)
• No regressions to relationship-derived subforms (master-detail inline editing)