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