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/iteration4a_rebuild_command.rs

@@ -131,8 +131,8 @@ fn rebuild_against_populated_db_wipes_and_reloads() {
             db.create_table(
                 "Customers".to_string(),
                 vec![
-                    ColumnSpec { name: "id".to_string(), ty: Type::Serial },
-                    ColumnSpec { name: "Name".to_string(), ty: Type::Text },
+                    ColumnSpec::new("id".to_string(), Type::Serial),
+                    ColumnSpec::new("Name".to_string(), Type::Text),
                 ],
                 vec!["id".to_string()],
                 Some("create".to_string()),