feat(hint): clause-concept hints layered on F1 (issue #37)

Add a hint.concept.* tier-3 layer surfaced when the cursor sits inside a
recognized clause (referential actions, 1:n/m:n cardinality, primary key,
unique, check, foreign key), layered beneath the per-form block. New
Node::Concept grammar wrapper records clause byte-spans; concept_topic_at_cursor
resolves the innermost containing span. Examples are mode-keyed so they stay
syntax-correct in both simple and advanced mode. Draft ADR (number at merge).

src/dsl/grammar/shared.rs

@@ -133,12 +133,25 @@ pub const ON_CLAUSE: Node = Node::Seq(ON_CLAUSE_NODES);
 /// Repeated `on <target> <action>` clauses (0..2 occurrences).
 /// Validation of "specified twice" + max=2 lives in the
 /// command's AST builder.
-pub const REFERENTIAL_CLAUSES: Node = Node::Repeated {
+const REFERENTIAL_CLAUSES_INNER: Node = Node::Repeated {
     inner: &ON_CLAUSE,
     separator: None,
     min: 0,
 };
 
+/// As [`REFERENTIAL_CLAUSES_INNER`], tagged as the
+/// `referential_actions` clause-concept region (issue #37).
+///
+/// Shared by the simple-mode `add 1:n relationship … on delete/update`
+/// and the advanced-mode `references … on delete/update` clause, so the
+/// one wrapper surfaces the concept in both modes (the mode-keyed
+/// example is chosen by the live mode at F1 time, not by which grammar
+/// matched).
+pub const REFERENTIAL_CLAUSES: Node = Node::Concept {
+    topic: "referential_actions",
+    inner: &REFERENTIAL_CLAUSES_INNER,
+};
+
 // =================================================================
 // Typed value slots (ADR-0024 §Phase D, §typed-value-slots)
 // =================================================================