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.

src/dsl/grammar/ddl.rs

@@ -616,6 +616,12 @@ fn build_add(path: &MatchedPath) -> Result<Command, ValidationError> {
                 table: require_ident(path, "table_name")?,
                 column: require_ident(path, "column_name")?,
                 ty,
+                // Constraint suffix is wired in once the
+                // constraint grammar lands (ADR-0029).
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             })
         }
         Some("1") => build_add_relationship(path),
@@ -907,10 +913,7 @@ fn build_create_table(path: &MatchedPath) -> Result<Command, ValidationError> {
 
     let columns = pk_specs
         .iter()
-        .map(|(n, t)| ColumnSpec {
-            name: n.clone(),
-            ty: *t,
-        })
+        .map(|(n, t)| ColumnSpec::new(n.clone(), *t))
         .collect();
     let primary_key = pk_specs.into_iter().map(|(n, _)| n).collect();