docs: define the interface pattern language

docs/FRONTEND_LAYOUT_PRINCIPLES.md

@@ -3,6 +3,11 @@
 GovOPlaN modules should choose their page layout by the kind of work the user is
 doing, not by the repository that owns the feature.
 
+These concise layout choices are one canonical input to the broader
+[`INTERFACE_PATTERN_LANGUAGE.md`](INTERFACE_PATTERN_LANGUAGE.md). The current
+route and rollout evidence lives in
+[`INTERFACE_SURFACE_INVENTORY.md`](INTERFACE_SURFACE_INVENTORY.md).
+
 ## Structured Data Directories
 
 Use a full-available-space workspace for structured data directories: files,
@@ -41,6 +46,9 @@ Principles:
   summaries, and dashboard widgets.
 - Collapsible panels and segmented controls are appropriate when a dense
   configuration area needs controlled disclosure.
+- A collapsible card whose sole content is a table gives that table the full
+  available card body; avoid nested cards, duplicate padding, inner max-widths,
+  and nested scrolling.
 - Avoid forcing workflow/configuration pages into a file-explorer style unless
   the primary interaction is genuinely directory browsing.