Publish share-selection group membership in capabilities

GET /api/meta/capabilities dropped group membership at the wire: it
shipped a flat group-id list and a flat leaf list with no boundary
between them, so a native client could not tell which leaves live
under which group, or which group is the fenced sensitive tier. That
made it unsafe for the client to offer a "select all" affordance for
any group, since it could not mirror the server's fence against
bundling more than one sensitive leaf behind a single control.

Add SHARE_GROUPS to the report-selection catalogue, derived from the
existing REPORT_GROUPS/MEASUREMENT_LEAF_GROUP/STRUCTURED_LEAF_GROUP
maps: each group now carries its member leaf ids, and the group whose
id equals SENSITIVE_GROUP_ID carries sensitive: true. share.groups in
the capabilities response now ships this shape instead of bare ids;
share.leaves is unchanged. Update the OpenAPI schema and regenerate
docs/api/openapi.yaml.

Add an invariant test asserting every leaf in the flat leaves set
appears in exactly one group and no group carries a leaf outside that
set, plus a check that exactly one group is flagged sensitive.

docs/api/openapi.yaml

@@ -16096,8 +16096,24 @@ components:
             groups:
               type: array
               items:
-                type: string
-              description: Selection group ids, in presentation order.
+                type: object
+                properties:
+                  id:
+                    type: string
+                    description: Group id.
+                  leaves:
+                    type: array
+                    items:
+                      type: string
+                    description: Member leaf ids, in the order the panel renders them.
+                  sensitive:
+                    description: "Present and true only on the fenced tier: no single control may switch on more than one of its leaves."
+                    type: boolean
+                required:
+                  - id
+                  - leaves
+                additionalProperties: false
+              description: Selection groups, in presentation order, each carrying its member leaf ids.
             leaves:
               type: array
               items: