grammar+db: 3i — not_null_missing diagnostic + TableColumn constraints (ADR-0033 §8.3)

Extend SchemaCache TableColumn with not_null + has_default (with a
TableColumn::new constructor for the common no-constraint case),
populated in build_schema_cache from ColumnDescription (a PK column
counts as not-null). New dml_not_null_missing_diagnostics pass: a
WARNING when a SQL INSERT's explicit column list omits a column that
is NOT NULL with no DEFAULT — advisory (the engine enforces it).
serial/shortid (auto-filled) and defaulted columns are excluded.
Anchored on the target-table ident (no token for the omitted column).

Catalog key diagnostic.not_null_missing (engine-neutral). Tests (+4):
fires on omitted required column; silent when included, when
defaulted, and for auto-gen serial/shortid. ~24 TableColumn literal
sites updated for the two new fields (build clean). 1591 pass / 0
fail / 1 ignored. Clippy clean.

All three ADR-0033 §8 DML diagnostics now implemented. Remaining 3i:
cross-cut verification + #12 UPSERT DO UPDATE validation.

src/dsl/walker/driver.rs

@@ -2035,9 +2035,9 @@ mod tests {
         s.table_columns.insert(
             "users".to_string(),
             vec![
-                TableColumn { name: "id".to_string(), user_type: Type::Int },
-                TableColumn { name: "name".to_string(), user_type: Type::Text },
-                TableColumn { name: "age".to_string(), user_type: Type::Int },
+                TableColumn { name: "id".to_string(), user_type: Type::Int, not_null: false, has_default: false },
+                TableColumn { name: "name".to_string(), user_type: Type::Text, not_null: false, has_default: false },
+                TableColumn { name: "age".to_string(), user_type: Type::Int, not_null: false, has_default: false },
             ],
         );
         s