[SDUI][P0] Wire PageDesigner → record page persistence (draft/publish) #1541

Context

Part of the SDUI runtime roadmap. The rendering engine (SchemaRenderer + registry) and record-detail synthesis (buildDefaultPageSchema) are done, and runtime draft→publish persistence already works for views/reports/dashboards via the ADR-0034 seam (packages/app-shell/src/views/runtime-metadata-persistence.ts). The PageDesigner (packages/plugin-designer/src/PageDesigner.tsx) can visually compose a component tree but its output is not persisted for record pages.

Goal

Close the write-side loop for record pages: edit a record PageSchema visually in the browser and save/publish it through the same draft/publish seam.

Scope

• Add 'page' (record PageSchema) as an artifact type to runtime-metadata-persistence.ts (persist/read/discard/publish), mirroring view/report/dashboard.
• Wire PageDesigner output → persistRuntimeMetadata('page', ...).
• Resolve the assigned page on load (usePageAssignment) including unpublished drafts (readRuntimeDraft).
• Round-trip: assign → edit → draft → publish → re-render via SchemaRenderer.

Acceptance

• Editing a record page in the browser persists a draft (invisible to others)
• Publish promotes the draft to the active page + version bump
• Discard reverts to the previous published/synthesized page
• Live e2e: open a record, edit its page, publish, reload → change persists

Notes

Dashboards/reports are the working reference pattern. Pairs with #1542 (RuntimeDraftBar).