[SDUI][P2] Home page rendering + assignment + default synthesis

[xuyushun441-sys]

Context

The home page type is registered in the component registry but has no renderer wiring, no default synthesis, and no assignment logic — it's effectively a stub.

Goal

Make the Home page a first-class SDUI surface: synthesized default + author-assignable + rendered via SchemaRenderer.

Scope

• usePageAssignment support for pageType: 'home'.
• A buildDefaultHomePageSchema (recent items, favorites, KPI widgets, quick actions).
• Render via PageRenderer/SchemaRenderer; assignable + editable via PageDesigner (depends on page persistence).

Acceptance

• Home renders a synthesized default when none assigned
• An assigned home PageSchema overrides it
• Editable + publishable once page persistence lands