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/data.rs

@@ -316,7 +316,7 @@ const DELETE_SHAPE: Node = Node::Seq(DELETE_NODES);
 
 fn ident_text<'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,
     })
 }
@@ -450,7 +450,7 @@ fn build_insert(path: &MatchedPath) -> Result<Command, ValidationError> {
     let table_idx = path
         .items
         .iter()
-        .position(|i| matches!(&i.kind, MatchedKind::Ident { role: "table_name" }))
+        .position(|i| matches!(&i.kind, MatchedKind::Ident { role: "table_name", .. }))
         .ok_or_else(|| ValidationError {
             message_key: "parse.error_wrapper",
             args: vec![("detail", "missing table".to_string())],
@@ -498,6 +498,7 @@ fn build_insert(path: &MatchedPath) -> Result<Command, ValidationError> {
             .filter_map(|i| match &i.kind {
                 MatchedKind::Ident {
                     role: "insert_first_item",
+                    ..
                 } => Some(i.text.clone()),
                 _ => None,
             })
@@ -551,6 +552,7 @@ fn build_insert(path: &MatchedPath) -> Result<Command, ValidationError> {
             .filter_map(|i| match &i.kind {
                 MatchedKind::Ident {
                     role: "insert_first_item",
+                    ..
                 } => Some(i.text.clone()),
                 _ => None,
             })
@@ -619,7 +621,8 @@ fn collect_assignments(
         if matches!(
             item.kind,
             MatchedKind::Ident {
-                role: "update_set_column"
+                role: "update_set_column",
+                ..
             }
         ) {
             let column = item.text.clone();