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/parser.rs

@@ -358,10 +358,7 @@ mod tests {
     }
 
     fn col(name: &str, ty: Type) -> ColumnSpec {
-        ColumnSpec {
-            name: name.to_string(),
-            ty,
-        }
+        ColumnSpec::new(name, ty)
     }
 
     #[test]
@@ -657,6 +654,10 @@ mod tests {
                 table: "Customers".to_string(),
                 column: "Name".to_string(),
                 ty: Type::Text,
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             }
         );
     }
@@ -671,6 +672,10 @@ mod tests {
                     table: "T".to_string(),
                     column: "C".to_string(),
                     ty: *ty,
+                    not_null: false,
+                    unique: false,
+                    default: None,
+                    check: None,
                 }
             );
         }
@@ -684,6 +689,10 @@ mod tests {
                 table: "Customers".to_string(),
                 column: "Name".to_string(),
                 ty: Type::Text,
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             }
         );
     }
@@ -696,6 +705,10 @@ mod tests {
                 table: "Customers".to_string(),
                 column: "Name".to_string(),
                 ty: Type::Text,
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             }
         );
     }
@@ -708,6 +721,10 @@ mod tests {
                 table: "Customers".to_string(),
                 column: "Name".to_string(),
                 ty: Type::Text,
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             }
         );
     }
@@ -720,6 +737,10 @@ mod tests {
                 table: "T".to_string(),
                 column: "Name".to_string(),
                 ty: Type::Text,
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             }
         );
     }