Adopt shared WebUI structural primitives

docs/INTERFACE_PATTERN_MIGRATION.md

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+# Docs Interface Pattern Migration
+
+The Docs route composes the platform interface language without owning a
+parallel shell. Core owns its navigation/content pane geometry and page frame;
+Docs owns audience filtering, version selection, topic navigation, configured
+documentation projection, and evidence presentation.
+
+## Surface contract
+
+- `WorkspaceLayout` owns the outline/content regions, pane width, scrolling,
+  accessible labels, and narrow-layout navigation behavior.
+- `PageLayout` owns the content inset, sticky heading, audience-aware page
+  identity, reload action placement, error and loading regions, and responsive
+  action flow.
+- `ExplorerTree`, `SegmentedControl`, `DataGrid`, `DescriptionList`, dialogs,
+  alerts, and status badges retain their Core interaction and accessibility
+  contracts.
+- Docs-specific CSS remains limited to the documentation tree, reading column,
+  topic typography, page outline, and reference content. It no longer defines
+  the outer workspace grid or page skeleton.
+
+The module keeps user/admin audience boundaries, installed-version selection,
+topic URLs, configured-state projection, and evidence visibility unchanged.
+The platform layout checker rejects restoring a raw Docs page or workspace
+frame.