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/input_render.rs

@@ -1344,6 +1344,39 @@ mod tests {
         }
     }
 
+    #[test]
+    fn advanced_select_genuine_column_typo_before_from_warns_at_typing_time() {
+        // Issue #16: the gap the issue-#6 trade-off opened. While the
+        // user types `select Agx` (no FROM yet, so the schema-existence
+        // diagnostic stays silent), a genuine column typo must warn at
+        // typing time via the restored `invalid_ident` path — `Agx`
+        // matches neither a schema column nor a known function name.
+        use crate::completion::{SchemaCache, TableColumn};
+        use crate::dsl::types::Type;
+        let mut cache = SchemaCache::default();
+        cache.tables.push("Customers".to_string());
+        let tc = vec![TableColumn {
+            name: "Age".to_string(),
+            user_type: Type::Int,
+            not_null: false,
+            has_default: false,
+        }];
+        for c in &tc {
+            cache.columns.push(c.name.clone());
+        }
+        cache.table_columns.insert("Customers".to_string(), tc);
+        let input = "select Agx";
+        match ambient_hint_in_mode(input, input.len(), None, &cache, Mode::Advanced) {
+            Some(AmbientHint::Prose(p)) => assert!(
+                p.contains("No such") && p.contains("Agx"),
+                "a genuine column typo before FROM must warn at typing time; got: {p:?}",
+            ),
+            other => panic!(
+                "`select Agx` must surface a typing-time typo hint; got: {other:?}",
+            ),
+        }
+    }
+
     #[test]
     fn advanced_partial_typing_does_not_leak_bare_double_in_prose() {
         // Issue #5 (prose half): at `create table Orders (count` (no