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/walker/driver.rs

@@ -290,6 +290,33 @@ fn walk_node_inner(
             ctx.pending_hint_mode = Some(*mode);
             walk_node(source, pos, inner, ctx, path, per_byte)
         }
+        Node::Concept { topic, inner } => {
+            // Issue #37 / ADR clause-concept-hints D1. Walk the
+            // inner clause, then record its covered byte span in
+            // `ctx.concept_spans` iff the inner *committed* — i.e.
+            // matched, ran out mid-clause (Incomplete), or hit a
+            // hard failure after engaging (Failed). On `NoMatch`
+            // the node never engaged (e.g. a `Choice` tried this
+            // branch and it didn't apply), so record nothing — no
+            // stale span. `start` is the post-whitespace position
+            // the walk wrapper already resolved.
+            let result = walk_node(source, pos, inner, ctx, path, per_byte);
+            let end = match &result {
+                NodeWalkResult::Matched { end, .. } => Some(*end),
+                NodeWalkResult::Incomplete { position, .. }
+                | NodeWalkResult::Failed { position, .. } => Some(*position),
+                NodeWalkResult::NoMatch { .. } => None,
+            };
+            if let Some(end) = end {
+                ctx.concept_spans
+                    .push(crate::dsl::walker::context::ConceptSpan {
+                        topic,
+                        start: pos,
+                        end,
+                    });
+            }
+            result
+        }
         Node::Flag(name) => walk_flag(source, pos, name, path, per_byte),
         Node::Repeated {
             inner,