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/requirements.md

@@ -26,18 +26,21 @@ repo is pushed).
 
 ## Test baseline
 
-After the ADR-0027 highlight / hint follow-up (precise WARNING
-spans, the diagnostic overlay + hint wiring, the
-`LIKE`-on-numeric WARNING, the debounce state machine) plus
-two manual-testing bug fixes (optional trailing-flag
-completion; the `--resume` temp-project pointer):
-**1131 passing, 0 failing, 1 ignored** (`cargo test` — the one
-ignored test is a long-standing `` ```ignore `` doc-test in
-`src/friendly/mod.rs`). Clippy clean with the nursery lint
-group enabled. (Earlier reference points: 1100 after ADR-0027's
-initial ship; 1079 after ADR-0026 (complex WHERE expressions);
-1039 after ADR-0025 (indexes); 1006 after ADR-0024 + the
-handoff-14 cleanup; 449 after B2/C2.)
+After ADR-0022 Amendment 6 (the curated SQL function-name list —
+issues #15 tab-completion + #16 typing-time typo hint):
+**1538 lib unit tests passing, 0 failing, 1 ignored**
+(`cargo test --lib`; the full `cargo test` across every binary is
+2107 passing, 0 failing, 1 ignored — the one ignored is a
+long-standing `` ```ignore `` doc-test in `src/friendly/mod.rs`).
+Clippy clean with the nursery lint group enabled. (Earlier
+reference points — lib counts: 1131 after the ADR-0027 highlight /
+hint follow-up + the optional-trailing-flag / `--resume`
+manual-testing fixes; 1100 after ADR-0027's initial ship; 1079
+after ADR-0026 (complex WHERE expressions); 1039 after ADR-0025
+(indexes); 1006 after ADR-0024 + the handoff-14 cleanup; 449
+after B2/C2. Note the intervening issue fixes #8/#13/#12/#7/#9
+landed tests without a baseline bump; this is the first refresh
+since ADR-0027.)
 
 ---
 
@@ -105,11 +108,18 @@ handoff-14 cleanup; 449 after B2/C2.)
   rolling history is out of scope per OOS-6 / N4.)*
 - [ ] **I3** Tab completion for app commands, DSL keywords, table
   names, column names, and SQL keywords.
+  *(Refinement 2026-05-30, issue #15: SQL expression slots
+  (`sql_expr_ident`) now also offer a curated set of SQL function
+  names — `KNOWN_SQL_FUNCTIONS` in `src/dsl/sql_functions.rs`,
+  surfaced as a new `CandidateKind::Function` — ADR-0022 Amendment 6.
+  The broad tab-completion goal stays open.)*
 - [ ] **I4** Syntax highlighting for both the DSL and SQL.
   *(Refinement 2026-05-29, issue #8: column data types now carry a
   dedicated `HighlightClass::Type` / `tok_type` colour, distinct from
-  identifiers and clause keywords — ADR-0022 Amendment 4. The broad
-  highlighting goal stays open.)*
+  identifiers and clause keywords — ADR-0022 Amendment 4; a further
+  refinement 2026-05-30, issue #15: SQL function-name candidates carry
+  a dedicated `tok_function` colour (the ninth `Theme` token colour,
+  ADR-0022 Amendment 6). The broad highlighting goal stays open.)*
 - [ ] **I5** In-flight query/command cancellation (Ctrl-C in the
   output area or input field).