constraints: CHECK — check (<expr>) at create table & add column (ADR-0029)

The fourth constraint. `check ( <expr> )` reuses the ADR-0026
WHERE-expression grammar via `Subgrammar`, so a check is
written in the same language as a `where` filter.

- Grammar: a `CHECK_CONSTRAINT` arm joins the shared
  constraint-suffix Choice; `consume_check_expr` extracts the
  parenthesised expression (paren-depth aware) into
  `ColumnSpec.check` / `Command::AddColumn.check`.
- Storage: the parsed `Expr` is compiled once to inline SQL
  (`compile_check_sql` — `compile_expr` + ADR-0028's
  param-inliner) and stored in that form everywhere — a new
  `check_expr` column in `__rdbms_playground_columns`,
  `project.yaml`'s `ColumnSchema.check`, and the column DDL
  emitted by `do_create_table` / `schema_to_ddl`.
- `add column … check` routes through the rebuild primitive
  (SQLite's `ALTER … ADD COLUMN` cannot carry it); a CHECK on
  a serial/shortid column is create-table-only and refused at
  add-column with a friendly message.
- `describe` surfaces the CHECK. ADR-0029 §7/§8 updated to the
  SQL-form decision — double-quoted identifiers, consistent
  with ADR-0028's `explain` display SQL.

1201 tests pass (+8); clippy clean.

src/persistence/csv_io.rs

@@ -376,6 +376,7 @@ mod tests {
             unique: false,
             not_null: false,
             default: None,
+            check: None,
         }
     }
 

src/persistence/mod.rs

@@ -152,6 +152,10 @@ pub struct ColumnSchema {
     /// form SQLite reports and `schema_to_ddl` echoes verbatim.
     /// `None` when the column has no default.
     pub default: Option<String>,
+    /// `CHECK` constraint in compiled-SQL form (ADR-0029 §7),
+    /// echoed verbatim into the rebuilt DDL. `None` when the
+    /// column has no check.
+    pub check: Option<String>,
 }
 
 /// One index as recorded in `project.yaml` (ADR-0025).
@@ -383,6 +387,7 @@ mod tests {
                 unique: false,
                 not_null: false,
                 default: None,
+                check: None,
             }],
             rows: vec![vec![CellValue::Text("Alice".to_string())]],
         };

src/persistence/yaml.rs

@@ -129,6 +129,10 @@ fn write_column(out: &mut String, col: &ColumnSchema) {
         line.push_str(", default: ");
         line.push_str(&yaml_string(default));
     }
+    if let Some(check) = &col.check {
+        line.push_str(", check: ");
+        line.push_str(&yaml_string(check));
+    }
     line.push_str(" }");
     let _ = writeln!(out, "{line}");
 }
@@ -238,6 +242,7 @@ pub(crate) fn parse_schema(body: &str) -> Result<SchemaSnapshot, YamlError> {
                 unique: c.unique,
                 not_null: c.not_null,
                 default: c.default,
+                check: c.check,
             });
         }
         tables.push(TableSchema {
@@ -370,6 +375,9 @@ struct RawColumn {
     /// `DEFAULT` SQL literal (ADR-0029); absent in older files.
     #[serde(default)]
     default: Option<String>,
+    /// `CHECK` SQL (ADR-0029); absent in older files.
+    #[serde(default)]
+    check: Option<String>,
 }
 
 #[derive(Deserialize)]
@@ -407,16 +415,16 @@ mod tests {
                     name: "Customers".to_string(),
                     primary_key: vec!["id".to_string()],
                     columns: vec![
-                        ColumnSchema { name: "id".to_string(), user_type: Type::Serial, unique: false, not_null: false, default: None },
-                        ColumnSchema { name: "Name".to_string(), user_type: Type::Text, unique: false, not_null: false, default: None },
+                        ColumnSchema { name: "id".to_string(), user_type: Type::Serial, unique: false, not_null: false, default: None, check: None },
+                        ColumnSchema { name: "Name".to_string(), user_type: Type::Text, unique: false, not_null: false, default: None, check: None },
                     ],
                 },
                 TableSchema {
                     name: "Orders".to_string(),
                     primary_key: vec!["id".to_string()],
                     columns: vec![
-                        ColumnSchema { name: "id".to_string(), user_type: Type::Serial, unique: false, not_null: false, default: None },
-                        ColumnSchema { name: "CustId".to_string(), user_type: Type::Int, unique: false, not_null: false, default: None },
+                        ColumnSchema { name: "id".to_string(), user_type: Type::Serial, unique: false, not_null: false, default: None, check: None },
+                        ColumnSchema { name: "CustId".to_string(), user_type: Type::Int, unique: false, not_null: false, default: None, check: None },
                     ],
                 },
             ],
@@ -484,6 +492,7 @@ mod tests {
                     unique: false,
                     not_null: false,
                     default: None,
+                    check: None,
                 }],
             }],
             relationships: vec![],
@@ -523,6 +532,7 @@ mod tests {
                         unique: false,
                         not_null: false,
                         default: None,
+                        check: None,
                     },
                     ColumnSchema {
                         name: "title".to_string(),
@@ -530,6 +540,7 @@ mod tests {
                         unique: true,
                         not_null: true,
                         default: Some("'untitled'".to_string()),
+                        check: None,
                     },
                     ColumnSchema {
                         name: "stock".to_string(),
@@ -537,6 +548,7 @@ mod tests {
                         unique: false,
                         not_null: false,
                         default: Some("0".to_string()),
+                        check: Some("\"stock\" >= 0".to_string()),
                     },
                 ],
             }],
@@ -622,8 +634,8 @@ relationships:
                 name: "Items".to_string(),
                 primary_key: vec!["a".to_string(), "b".to_string()],
                 columns: vec![
-                    ColumnSchema { name: "a".to_string(), user_type: Type::Int, unique: false, not_null: false, default: None },
-                    ColumnSchema { name: "b".to_string(), user_type: Type::Int, unique: false, not_null: false, default: None },
+                    ColumnSchema { name: "a".to_string(), user_type: Type::Int, unique: false, not_null: false, default: None, check: None },
+                    ColumnSchema { name: "b".to_string(), user_type: Type::Int, unique: false, not_null: false, default: None, check: None },
                 ],
             }],
             relationships: vec![],