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/project_lifecycle.rs

@@ -165,14 +165,8 @@ fn db_persists_across_open_close_cycles() {
             db.create_table(
                 "Customers".to_string(),
                 vec![
-                    rdbms_playground::dsl::ColumnSpec {
-                        name: "id".to_string(),
-                        ty: rdbms_playground::dsl::Type::Serial,
-                    },
-                    rdbms_playground::dsl::ColumnSpec {
-                        name: "Name".to_string(),
-                        ty: rdbms_playground::dsl::Type::Text,
-                    },
+                    rdbms_playground::dsl::ColumnSpec::new("id".to_string(), rdbms_playground::dsl::Type::Serial),
+                    rdbms_playground::dsl::ColumnSpec::new("Name".to_string(), rdbms_playground::dsl::Type::Text),
                 ],
                 vec!["id".to_string()],
             None)