// Package mysql provides a database/sql-backed MySQL client with lifecycle
// management, health checks, and unit-of-work transaction support for
// Einherjar applications.
//
// # Overview
//
// [New] returns a [Component] that manages a *sql.DB connection pool,
// satisfies the [lifecycle.Component] lifecycle hooks (OnInit, OnStart, OnStop),
// and implements [observability.Checkable] with critical priority.
//
// [NewUnitOfWork] wraps multiple repository operations in a single transaction
// via context injection — no transaction object is passed between functions.
//
// # Lifecycle Registration
//
//	db := mysql.New(logger, cfg)
//	launcher.Register(db)
//	health.Register(db)
//
// # Querying
//
// Repository code receives a [Provider] or [Executor] and calls [Provider.GetExecutor]
// to obtain the active transaction (if inside a [UnitOfWork]) or the pool:
//
//	exec := db.GetExecutor(ctx)
//	row := exec.QueryRowContext(ctx, "SELECT id FROM users WHERE email = ?", email)
//	if err := row.Scan(&id); err != nil {
//	    return db.HandleError(err)
//	}
//
// # Unit of Work
//
// [NewUnitOfWork] wraps operations in a single transaction. The transaction is
// injected into the context; [Provider.GetExecutor] returns it automatically.
//
//	uow := mysql.NewUnitOfWork(logger, db)
//	err := uow.Do(ctx, func(ctx context.Context) error {
//	    exec := db.GetExecutor(ctx) // returns active Tx
//	    _, err := exec.ExecContext(ctx, "INSERT INTO orders ...")
//	    return err
//	})
//
// # Error Handling
//
// [HandleError] translates MySQL driver errors into typed [xerrors] values.
// Call it at every point where a driver error is first observed:
//
//	if err := row.Scan(&out); err != nil {
//	    return db.HandleError(err)
//	}
//
// Mapped codes:
//   - 1062 (ER_DUP_ENTRY)                → ErrAlreadyExists
//   - 1216, 1217, 1451, 1452 (FK errors) → ErrInvalidInput
//   - sql.ErrNoRows                       → ErrNotFound
//   - all others                          → ErrInternal
package mysql