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/context.rs

@@ -134,6 +134,18 @@ pub struct WalkContext<'a> {
     /// resolver reads this directly instead of inferring the
     /// slot kind from the shape of the expected set.
     pub pending_hint_mode: Option<crate::dsl::grammar::HintMode>,
+    /// Byte spans of the clause-concept regions the walk passed
+    /// through (issue #37 / ADR clause-concept-hints, D1).
+    ///
+    /// A `Node::Concept { topic, inner }` wrapper pushes one entry
+    /// per *committed* inner walk (Matched / Incomplete / Failed —
+    /// never on NoMatch). Unlike `pending_hint_mode`, this is
+    /// **append-only and never cleared on match**, so a fully-typed
+    /// clause keeps its span — that is what lets the F1
+    /// clause-concept resolver report a concept for a cursor parked
+    /// *inside* already-typed clause text, not only at the slot
+    /// boundary.
+    pub concept_spans: Vec<ConceptSpan>,
     /// An `IntroProse` hint captured from an *optional* slot that
     /// the walk skipped (issue #26). Unlike `pending_hint_mode`
     /// (cleared on the very next match — including the empty match
@@ -227,6 +239,19 @@ pub struct PendingCteHarvest {
     pub cte_name_span: (usize, usize),
 }
 
+/// A clause-concept region recorded during a walk (issue #37).
+///
+/// `topic` is the `hint.concept.<topic>` catalogue key; `[start,
+/// end]` is the inclusive byte range the clause covered as typed so
+/// far (`end` is the matched end on a full match, or the stop
+/// position on an incomplete / failed inner).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ConceptSpan {
+    pub topic: &'static str,
+    pub start: usize,
+    pub end: usize,
+}
+
 impl<'a> WalkContext<'a> {
     /// Schemaless walk context — the legacy default used by
     /// pre-Phase-D callers and tests that don't care about
@@ -243,6 +268,7 @@ impl<'a> WalkContext<'a> {
             pending_value_type: None,
             pending_value_column: None,
             pending_hint_mode: None,
+            concept_spans: Vec::new(),
             surviving_intro_hint: None,
             user_listed_columns: None,
             subgrammar_depth: 0,
@@ -266,6 +292,7 @@ impl<'a> WalkContext<'a> {
             pending_value_type: None,
             pending_value_column: None,
             pending_hint_mode: None,
+            concept_spans: Vec::new(),
             surviving_intro_hint: None,
             user_listed_columns: None,
             subgrammar_depth: 0,