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/friendly/strings/en-US.yaml

@@ -504,6 +504,7 @@ diagnostic:
   # ADR-0033 §8 — Phase-3 DML diagnostic keys.
   auto_column_overridden: "column `{column}` is auto-generated (`{type}`); providing an explicit value bypasses the auto-counter and may collide with later auto-generated values"
   insert_arity_mismatch: "the column list names {expected} column(s) but {actual} value(s) are given"
+  not_null_missing: "column `{column}` is required (`NOT NULL`, no default); the statement will fail when run"
 
 # Engine-error translations: an engine-rejected SQL statement
 # reaches the friendly-error layer (ADR-0019) and these keys