walker: schema-existence ERROR diagnostics (ADR-0027 step B)

`MatchedKind::Ident` now carries its `IdentSource`. A
post-walk pass over a structurally-valid parse flags a
matched `Tables` ident that is absent from the schema, or a
`Columns` ident absent from the table in scope, as an ERROR
diagnostic — the command parses but would fail at execution
(ADR-0027 §2). New behaviour: an unknown table / column used
to parse cleanly and fail only when run.

Column scope is resolved by one left-to-right pass over the
matched path (every command places its table ident before
the columns that belong to it); an unknown table clears the
scope, so its columns are not cascaded into a second
diagnostic. New catalog keys `diagnostic.unknown_table` /
`diagnostic.unknown_column`.

src/dsl/grammar/ddl.rs

@@ -475,7 +475,7 @@ const CHANGE_COLUMN: Node = Node::Seq(CHANGE_COLUMN_NODES);
 /// First ident whose role matches.
 fn ident<'a>(path: &'a MatchedPath, role: &str) -> Option<&'a str> {
     path.items.iter().find_map(|i| match &i.kind {
-        MatchedKind::Ident { role: r } if *r == role => Some(i.text.as_str()),
+        MatchedKind::Ident { role: r, .. } if *r == role => Some(i.text.as_str()),
         _ => None,
     })
 }
@@ -495,7 +495,7 @@ fn collect_idents(path: &MatchedPath, role: &str) -> Vec<String> {
     path.items
         .iter()
         .filter_map(|i| match &i.kind {
-            MatchedKind::Ident { role: r } if *r == role => Some(i.text.clone()),
+            MatchedKind::Ident { role: r, .. } if *r == role => Some(i.text.clone()),
             _ => None,
         })
         .collect()
@@ -862,7 +862,7 @@ fn build_create_table(path: &MatchedPath) -> Result<Command, ValidationError> {
         .items
         .iter()
         .filter_map(|i| match &i.kind {
-            MatchedKind::Ident { role: "col_name" } => Some(i.text.clone()),
+            MatchedKind::Ident { role: "col_name", .. } => Some(i.text.clone()),
             _ => None,
         })
         .collect();
@@ -870,7 +870,7 @@ fn build_create_table(path: &MatchedPath) -> Result<Command, ValidationError> {
         .items
         .iter()
         .filter_map(|i| match &i.kind {
-            MatchedKind::Ident { role: "col_type" } => Some(i.text.as_str()),
+            MatchedKind::Ident { role: "col_type", .. } => Some(i.text.as_str()),
             _ => None,
         })
         .collect();