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/strings/en-US.yaml

@@ -458,6 +458,14 @@ parse:
     mode: "mode simple  |  mode advanced"
     messages: "messages  |  messages short  |  messages verbose"
 
+# ---- Pre-submit diagnostics (ADR-0027) -------------------------------
+# Surfaced by the validity indicator and the hint panel before
+# a command is submitted, so a learner sees the problem without
+# having to run the command and read the engine's error.
+diagnostic:
+  unknown_table: "no such table: `{name}`"
+  unknown_column: "no such column `{name}` on table `{table}`"
+
 # ---- Project lifecycle event notes -----------------------------------
 project:
   rebuild_ok: "[ok] rebuild — {summary}"