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.

tests/typing_surface/mod.rs

@@ -137,6 +137,8 @@ fn build_schema(tables: &[(&str, &[(&str, Type)])]) -> SchemaCache {
             .map(|(n, t)| TableColumn {
                 name: (*n).to_string(),
                 user_type: *t,
+                not_null: false,
+                has_default: false,
             })
             .collect();
         cache.tables.push((*table).to_string());