feat: centralize shared page layouts

docs/INTERFACE_PATTERN_MIGRATION.md

@@ -13,6 +13,7 @@ domain modules own their compositions.
 | Reusable credentials | Repeated administration with an adaptive create/edit dialog, optional password generator, and destructive confirmation | Secret values are write-only; generated candidates use the browser cryptographic API without a weak fallback and do not replace the field until explicitly confirmed; scope/permission blockers name the required action, responsible actor, and destination; unavailable row actions remain keyboard-explainable | `CredentialEnvelopeManager.tsx`, shared `PasswordField`, `PasswordGeneratorDialog`, `ActionBlockerHint`, `Button`, `TableActionGroup`, and `ConfirmDialog` |
 | Retention policy | Effective-policy editor with inherited source paths and typed, narrowing-only controls | Parent locks and missing write authority are explicit; the save action distinguishes locks, missing target, loading, clean draft, and active save | `RetentionPolicyManagement.tsx`, policy logic tests, `test-core-interface-patterns.mjs` |
 | Module lifecycle | Guided operator projection over durable installer-queue evidence | Preflight, handoff, progress, stale evidence, recovery, and rollback consequences remain visible | Admin module lifecycle tests and the Core installer-queue contract |
+| Shared page frame | Domain-neutral headed page layout used by Core and optional modules | Scroll ownership, sticky heading, route actions, page notices, loading, narrow-layout collapse, and contextual-help identity are centralized; `AdminPageLayout` composes the same contract | `PageLayout.tsx`, `page-layout.test.tsx`, Core fallback dashboard, Dashboard module, Ops module, and `check-shared-webui-layouts.py` |
 | Shared configuration primitives | Cross-module component contract | Dialog focus, blocker structure, disabled-action focus, route/page/field/action F1 help, unsaved changes, confirmation, loading, alerts, problem lists, and policy provenance are centralized | Core component tests, `CONTEXTUAL_HELP_CONTRACT.md`, and module-permutation build |
 
 ## Boundary
@@ -26,3 +27,8 @@ they are not reasons to add sibling-private behavior to Core.
 Raw JSON remains permitted only for diagnostics, expert inspection,
 interchange, or conflict evidence. It is not a primary Core configuration
 editor.
+
+Raw page-frame debt is registered in the meta repository and may only decrease.
+New headed pages use `PageLayout`; full-canvas explorers may use `PageHeader`
+while the next shared workspace/split-pane contract is developed. Module CSS
+continues to own domain content layout, never the shared page frame.