fix(completion): treat a bare in-scope table alias as an alias, not an unknown column (#31)

A bare table alias typed where a column is expected — `… GROUP BY o`,
with `o` aliasing `FROM Orders o` — was a blind spot: completion offered
nothing for `o`, and the hint panel called the in-scope alias an unknown
column (`no such column o on table Orders, ...`).

Completion now offers each FROM source's qualifier (alias-if-present-else
table-name) at a bare sql_expr_ident slot, folded into the column
candidate list; on an exact-qualifier partial the alias source steps
aside so the diagnostic can surface. The bare-reference diagnostic arm
emits a targeted `alias_used_as_column` / `table_used_as_column` hint
("`o` is a table alias — write `o.<column>` ...") after the
projection-alias check, so ORDER-BY alias refs still win and a genuine
unknown column still reports `unknown_column`.

Two guards keep the qualified-form advice correct: SQL only (role
`sql_expr_ident`, so the DSL `expr_column` path keeps `unknown_column`
since the DSL has no `table.column` syntax) and effective-qualifier
match (alias-if-present-else-table, so an aliased source referenced by
its shadowed real name falls through rather than being advised as
`name.<column>`). The diagnostic is a drop-in replacement for
`unknown_column` at the same span/Error severity, so verdict/overlay/hint
paths are unchanged.

ADR-0032 Amendment 3; +10 tests.

src/friendly/keys.rs

@@ -38,6 +38,7 @@
 /// `(key, expected_placeholders)`. Sorted by key for grep-ability.
 pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     // ---- Pre-submit diagnostics (ADR-0027) ----
+    ("diagnostic.alias_used_as_column", &["name"]),
     ("diagnostic.ambiguous_column", &["column", "qualifiers"]),
     ("diagnostic.auto_column_overridden", &["column", "type"]),
     ("diagnostic.compound_arity_mismatch", &["op", "left_n", "right_n"]),
@@ -63,6 +64,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("diagnostic.not_null_missing", &["column"]),
     ("diagnostic.like_numeric", &["column", "type"]),
     ("diagnostic.projection_alias_misplaced", &["alias", "clause"]),
+    ("diagnostic.table_used_as_column", &["name"]),
     ("diagnostic.type_mismatch", &["column", "type"]),
     ("diagnostic.unknown_column", &["name", "table"]),
     ("diagnostic.unknown_qualifier", &["qualifier"]),