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/walking_skeleton.rs

@@ -90,10 +90,7 @@ fn typing_then_submitting_a_dsl_command_emits_execute_action() {
         &actions,
         &Command::CreateTable {
             name: "Customers".to_string(),
-            columns: vec![ColumnSpec {
-                name: "id".to_string(),
-                ty: Type::Serial,
-            }],
+            columns: vec![ColumnSpec::new("id".to_string(), Type::Serial)],
             primary_key: vec!["id".to_string()],
         },
     );
@@ -271,10 +268,7 @@ fn create_table_flow_updates_tables_list_and_structure_view() {
     let actions = submit(&mut app);
     let expected_cmd = Command::CreateTable {
         name: "Customers".to_string(),
-        columns: vec![ColumnSpec {
-            name: "id".to_string(),
-            ty: Type::Serial,
-        }],
+        columns: vec![ColumnSpec::new("id".to_string(), Type::Serial)],
         primary_key: vec!["id".to_string()],
     };
     assert_one_execute_dsl(&actions, &expected_cmd);
@@ -326,6 +320,10 @@ fn add_column_flow_updates_structure_view() {
             table: "Customers".to_string(),
             column: "Name".to_string(),
             ty: Type::Text,
+            not_null: false,
+            unique: false,
+            default: None,
+            check: None,
         },
     );
 
@@ -338,6 +336,10 @@ fn add_column_flow_updates_structure_view() {
             table: "Customers".to_string(),
             column: "Name".to_string(),
             ty: Type::Text,
+            not_null: false,
+            unique: false,
+            default: None,
+            check: None,
         },
         description: Some(updated.clone()),
     });
@@ -479,6 +481,10 @@ fn add_relationship_flow_shows_inbound_section_on_parent() {
             table: "Customers".to_string(),
             column: "extra".to_string(),
             ty: Type::Text,
+            not_null: false,
+            unique: false,
+            default: None,
+            check: None,
         },
         description: Some(customers),
     });