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/runtime.rs

@@ -32,7 +32,7 @@ use crate::db::{
     AddColumnResult, ChangeColumnTypeResult, DataResult, Database, DbError, DeleteResult,
     DropColumnResult, InsertResult, QueryPlan, TableDescription, UpdateResult,
 };
-use crate::dsl::Command;
+use crate::dsl::{Command, ColumnSpec};
 use crate::dsl::walker::Severity;
 use crate::event::AppEvent;
 use crate::project::{
@@ -1706,8 +1706,27 @@ async fn execute_command_typed(
             .drop_table(name, src)
             .await
             .map(|()| CommandOutcome::Schema(None)),
-        Command::AddColumn { table, column, ty } => database
-            .add_column(table, column, ty, src)
+        Command::AddColumn {
+            table,
+            column,
+            ty,
+            not_null,
+            unique,
+            default,
+            check,
+        } => database
+            .add_column(
+                table,
+                ColumnSpec {
+                    name: column,
+                    ty,
+                    not_null,
+                    unique,
+                    default,
+                    check,
+                },
+                src,
+            )
             .await
             .map(CommandOutcome::AddColumn),
         Command::DropColumn {