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/friendly/keys.rs

@@ -37,6 +37,9 @@
 
 /// `(key, expected_placeholders)`. Sorted by key for grep-ability.
 pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
+    // ---- Pre-submit diagnostics (ADR-0027) ----
+    ("diagnostic.unknown_column", &["name", "table"]),
+    ("diagnostic.unknown_table", &["name"]),
     // ---- Already-exists collisions (anchor: "already exists") ----
     ("error.already_exists.column.headline", &["table", "column"]),
     ("error.already_exists.relationship.headline", &["name"]),