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/walker/driver.rs

@@ -434,7 +434,7 @@ fn walk_ident(
             .push(canonical);
     }
     path.push(MatchedItem {
-        kind: MatchedKind::Ident { role },
+        kind: MatchedKind::Ident { role, source: src },
         text,
         span: (start, end),
     });