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.

docs/adr/0031-sql-expression-grammar.md

@@ -409,3 +409,24 @@ Later phases extend the same fragment:
   set the engine-neutrality posture and the no-allowlist rule.
 - `docs/simple-mode-limitations.md` — the DSL limits this grammar
   lifts for advanced mode (§1, §4).
+
+## Status note — known-function list layered on the slot (2026-05-30)
+
+The `sql_expr_ident` slot is `IdentSource::Columns` and, per §1 / §5,
+does **not** itself know which identifiers are function names — it
+optimises for the common case (a column reference) and admits the
+function-call shape structurally; §5 explicitly noted "function names
+are not completed … a typed function name simply is not a candidate".
+**ADR-0022 Amendment 6** layers a curated known-function list
+(`src/dsl/sql_functions.rs`) on top of this slot, consumed two ways:
+as Tab-completion candidates so a learner can discover `sum` / `upper`
+/ … (issue #15 — softening §5's "not completed" line to "completed
+from a curated pedagogical list, not an allowlist for validation"),
+and as the allow-list that lets the typing-time column-typo hint stay
+strict at this slot — flag a partial as "no such column" only when it
+matches neither a schema column nor a known function name (issue #16).
+The grammar here is unchanged, and §6/§7's no-validation-allowlist
+posture stands: the list drives completion + the typo hint, **not**
+parse-time acceptance (an unknown function still parses and surfaces an
+engine-neutral execution error). The list sits in the completion /
+hint layer above the grammar.