feat: curated SQL function list — Tab completion (#15) + typing-time typo hint (#16)

Add src/dsl/sql_functions.rs (KNOWN_SQL_FUNCTIONS) as the shared source
of truth at sql_expr_ident slots:

- #15: offer the functions as Tab candidates under a new
  CandidateKind::Function + ninth Theme colour tok_function (blue,
  distinct from keyword/identifier/type).
- #16: restore the column-typo flag the #6 fix had dropped wholesale —
  invalid_ident_at_cursor now bails only when the partial prefix-matches
  a known function, else falls through to the schema-column check.

A column named like a function (e.g. `count`) is deduped (column wins).
`cast` is excluded — CAST(x AS type) is not a plain-call shape.
The no-validation-allowlist posture stands: the list drives completion +
the typo hint only, never parse-time acceptance.

Docs: ADR-0022 Amendment 6, ADR-0031 status note, README index,
requirements I3/I4 + refreshed test baseline.

src/ui.rs

@@ -1136,6 +1136,7 @@ fn render_candidate_line(
             crate::completion::CandidateKind::Identifier => theme.tok_identifier,
             crate::completion::CandidateKind::Flag => theme.tok_flag,
             crate::completion::CandidateKind::Punct => theme.tok_punct,
+            crate::completion::CandidateKind::Function => theme.tok_function,
         };
         let base_fg = if mixed {
             match items[i].mode {