refactor: ColumnSpec / AddColumn carry constraint fields (ADR-0029 scaffolding)

Expand ColumnSpec and Command::AddColumn with the four
ADR-0029 constraint slots (not_null, unique, default, check),
all defaulting off; `Database::add_column` now takes a
ColumnSpec. No behaviour change — the grammar to set the
fields and the DDL to enforce them land in the following
commits. Isolated here so those commits stay readable.

Adds ColumnSpec::new for the unconstrained case; 110 call
sites updated. 1172 tests pass; clippy clean.

tests/iteration4b_lifecycle_commands.rs

@@ -388,7 +388,7 @@ fn temp_with_a_table_is_no_longer_unmodified() {
     rt.block_on(async {
         db.create_table(
             "T".to_string(),
-            vec![ColumnSpec { name: "id".to_string(), ty: Type::Serial }],
+            vec![ColumnSpec::new("id".to_string(), Type::Serial)],
             vec!["id".to_string()],
             Some("create".to_string()),
         )