fix: X4 — advanced-mode SQL INSERT auto-fills omitted non-PK serial (MAX+1)

A Form-A advanced-mode INSERT that omitted a non-PK serial column left it
silently NULL (the column is INTEGER UNIQUE, not NOT NULL, so SQLite
permits it), while simple-mode do_insert auto-fills it with MAX+1. That
violated ADR-0018 §1's "auto-generated on every path" contract and was the
unprincipled serial-vs-shortid asymmetry the ADR set out to remove
(advanced mode already auto-fills shortid).

Fix (decision: advanced mode matches simple mode): the advanced-mode
auto-fill reconstruction — renamed plan_shortid_autofill →
plan_autogen_autofill — now also fills an omitted non-PK serial with
MAX(col)+1 … MAX+n per row (single- and multi-row), reading MAX once under
the worker's single-writer serialisation. PK serial stays on the rowid
alias; Form B (no column list) still supplies every column. Honours
ADR-0018 §1/§5; no ADR amendment needed (the contract already said "every
path"). requirements.md X4 marked resolved.

Tests: 1949 passing (+1), 0 failed, 0 skipped, 1 ignored; clippy clean.

tests/sql_insert.rs

@@ -707,6 +707,37 @@ fn autofill_does_not_mask_arity_mismatch() {
     assert!(rows.is_empty(), "no row should land on a rejected insert: {rows:?}");
 }
 
+#[test]
+fn sql_insert_autofills_omitted_nonpk_serial() {
+    // ADR-0018 §1/§5 + X4: advanced-mode SQL INSERT auto-fills an omitted
+    // non-PK `serial` column with MAX+1 (per row), exactly as simple-mode
+    // `do_insert` does — honouring the "auto-generated on every path"
+    // contract. (Was: silently inserting NULL.) Mirrors the existing
+    // shortid auto-fill, which already runs on this path.
+    let (project, db, _dir) = open_project_db();
+    let rt = rt();
+    create_cols(&db, &rt, "t", &[("id", Type::Int), ("seq", Type::Serial)], &["id"]);
+
+    // Single row, omitting the non-PK serial `seq`.
+    run_sqlinsert(&db, &rt, "insert into t (id) values (10)").expect("single-row insert runs");
+    // Multi-row, omitting `seq` — each row gets a distinct, increasing
+    // serial continuing from the current MAX.
+    run_sqlinsert(&db, &rt, "insert into t (id) values (20), (30)")
+        .expect("multi-row insert runs");
+
+    let rows = csv_rows(&project, "t");
+    // No NULL serials, and the sequence is 1, 2, 3 across the three rows.
+    assert_eq!(
+        rows,
+        vec![
+            vec!["10".to_string(), "1".to_string()],
+            vec!["20".to_string(), "2".to_string()],
+            vec!["30".to_string(), "3".to_string()],
+        ],
+        "omitted non-PK serial auto-filled MAX+1 per row (no NULLs): {rows:?}",
+    );
+}
+
 #[test]
 fn autofill_insert_select_wider_projection_is_rejected() {
     // SELECT projects more columns than the list: the guard defers