db+walker: 3i DA pass — not_null PK false-positive fix + arity hardening

DA pass on 3i. Fix: build_schema_cache set not_null = c.notnull ||
c.primary_key, which would false-flag an omitted `int` PK as a
not_null_missing WARNING — but an int PK is an INTEGER PRIMARY KEY
rowid alias that auto-fills (and SQLite's PK-NULL quirk means a PK
isn't implicitly NOT NULL anyway). Use c.notnull alone (ADR-0033
§8.3 "declared NOT NULL"): faithful and false-positive-free.

Arity-walk hardening (same class as the ON CONFLICT regression the
existing tests caught mid-3i): RETURNING after VALUES is a depth-0
keyword that ends the tuple list (only the real tuple is flagged),
and a comma nested in a function-call value (depth ≥ 2) does not
inflate the tuple's value count.

Tests (+2). 1598 pass / 0 fail / 1 ignored. Clippy clean.

src/dsl/walker/mod.rs

@@ -4554,6 +4554,29 @@ mod tests {
         );
     }
 
+    #[test]
+    fn insert_arity_with_returning_flags_only_tuple() {
+        // RETURNING is a depth-0 keyword that ends the VALUES clause;
+        // the walk must stop there (same guard as ON CONFLICT) so the
+        // RETURNING projection isn't mis-counted as a value tuple.
+        let schema = schema_with("t", &[("a", Type::Int), ("b", Type::Int)]);
+        let diags = diag_keys("sqlinsert into t (a, b) values (1, 2, 3) returning *", &schema);
+        let n = diags.iter().filter(|d| d.contains("value(s) are given")).count();
+        assert_eq!(n, 1, "only the 3-value tuple is flagged; got {diags:?}");
+    }
+
+    #[test]
+    fn insert_arity_ignores_commas_nested_in_a_value() {
+        // A comma inside a function call (depth ≥ 2) is not a tuple
+        // separator and must not inflate the value count.
+        let schema = schema_with("t", &[("a", Type::Int), ("b", Type::Int)]);
+        let diags = diag_keys("sqlinsert into t (a, b) values (1, coalesce(1, 2))", &schema);
+        assert!(
+            !diags.iter().any(|d| d.contains("value(s) are given")),
+            "two values (one a 2-arg call) match the 2-col list; got {diags:?}",
+        );
+    }
+
     #[test]
     fn insert_select_arity_mismatch_fires() {
         // INSERT … SELECT: 1 listed column vs a 2-item projection.