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/command.rs

@@ -15,13 +15,42 @@ use crate::dsl::action::ReferentialAction;
 use crate::dsl::types::Type;
 use crate::dsl::value::Value;
 
-/// A column at table-creation time: a name and a user-facing
-/// type. Constraints beyond `PRIMARY KEY` (NOT NULL, UNIQUE,
-/// CHECK, DEFAULT) come in later iterations.
+/// A column at table-creation time: a name, a user-facing
+/// type, and its column-level constraints (ADR-0029).
+///
+/// `PRIMARY KEY` is not represented here — it is a table-level
+/// property carried separately in `Command::CreateTable`'s
+/// `primary_key` list, since it may span columns.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ColumnSpec {
     pub name: String,
     pub ty: Type,
+    /// `NOT NULL` — the column rejects `NULL` (ADR-0029).
+    pub not_null: bool,
+    /// `UNIQUE` — non-`NULL` values must be distinct (ADR-0029).
+    pub unique: bool,
+    /// `DEFAULT <literal>` — the value used when an `insert`
+    /// omits this column (ADR-0029).
+    pub default: Option<Value>,
+    /// `CHECK (<expr>)` — every row must satisfy this boolean
+    /// expression (ADR-0029).
+    pub check: Option<Expr>,
+}
+
+impl ColumnSpec {
+    /// A column spec carrying no constraints — the common case
+    /// for callers and tests that do not exercise ADR-0029.
+    #[must_use]
+    pub fn new(name: impl Into<String>, ty: Type) -> Self {
+        Self {
+            name: name.into(),
+            ty,
+            not_null: false,
+            unique: false,
+            default: None,
+            check: None,
+        }
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -39,10 +68,18 @@ pub enum Command {
     DropTable {
         name: String,
     },
+    /// Add a column to an existing table. The column carries
+    /// its constraints from the same suffix grammar as
+    /// `create table` (ADR-0029); `check` is `None` until the
+    /// CHECK grammar lands.
     AddColumn {
         table: String,
         column: String,
         ty: Type,
+        not_null: bool,
+        unique: bool,
+        default: Option<Value>,
+        check: Option<Expr>,
     },
     /// Remove a column from a table. Refused if the column is
     /// part of the primary key or is involved in a declared

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();
 

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,
             }
         );
     }

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]