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/walker/mod.rs

@@ -1346,6 +1346,10 @@ mod tests {
                 table: "Customers".to_string(),
                 column: "Email".to_string(),
                 ty: Type::Text,
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             }
         );
     }
@@ -1474,10 +1478,7 @@ mod tests {
     use crate::dsl::command::ColumnSpec;
 
     fn col(name: &str, ty: Type) -> ColumnSpec {
-        ColumnSpec {
-            name: name.to_string(),
-            ty,
-        }
+        ColumnSpec::new(name, ty)
     }
 
     #[test]