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/expr.rs

@@ -525,7 +525,7 @@ impl<'a> ExprParser<'a> {
             .advance()
             .ok_or_else(|| drift_error("expected an operand"))?;
         match &item.kind {
-            MatchedKind::Ident { role: "expr_column" } => {
+            MatchedKind::Ident { role: "expr_column", .. } => {
                 Ok(Operand::Column(item.text.clone()))
             }
             MatchedKind::Word("null") => Ok(Operand::Literal(Value::Null)),