feat(db-mysql): initial implementation — database/sql MySQL with lifecycle and UnitOfWork (v1.0.0)

Introduces code.nochebuena.dev/einherjar/db-mysql — the MySQL database starter
for the Einherjar framework. Absorbs the mysql package from micro-lib using the
go-sql-driver/mysql driver, replacing fmt.Errorf wrapping with core/xerrors.

Interfaces (CT-6: one TypeSpec per file):
- Executor — ExecContext, QueryContext, QueryRowContext (database/sql types)
- Tx — Executor + Commit(), Rollback() — no ctx; honest database/sql contract
- Provider — GetExecutor, Begin, BeginTx, Ping, HandleError
- Component — lifecycle.Component + observability.Checkable + Provider + Stats()
- UnitOfWork — Do(ctx, fn)

Implementation:
- New(logger, cfg) Component — pool not created until OnInit
- OnInit: sql.Open + SetMaxOpenConns/SetMaxIdleConns/SetConnMaxLifetime/SetConnMaxIdleTime
- OnStart: Ping with 5s timeout; logs "mysql: connected"
- OnStop: db.Close(); logs "mysql: closing pool"
- GetExecutor: returns active Tx from context (ctxTxKey) or *sql.DB; explicit
  nil return when db uninitialized to avoid typed-nil interface pitfall
- Begin/BeginTx: wraps db.BeginTx(*sql.TxOptions); wrapped in xerrors on error
- HealthCheck: delegates to Ping; Priority LevelCritical
- Stats() sql.DBStats — zero value when pool uninitialized
- NewUnitOfWork(logger, provider) UnitOfWork — Begin+inject+commit/rollback;
  no write mutex (MySQL is a server DB with internal concurrency control)
- HandleError: 1062→ErrAlreadyExists, 1216/1217/1451/1452→ErrInvalidInput,
  sql.ErrNoRows→ErrNotFound, all others→ErrInternal
- Driver imported as mysqldrv alias in errors.go to avoid package-name collision;
  registered via blank import in new.go

Config (EINHERJAR_MYSQL_* env vars):
  Host, Port(3306), User, Password, Name,
  MaxConns(5), MinConns(2), MaxConnLifetime(1h), MaxConnIdleTime(30m),
  Charset(utf8mb4), Loc(UTC), ParseTime(true)

- Component interface embeds observability.Identifiable; identifiable.go implements
  ModulePath and ModuleVersion via runtime/debug.ReadBuildInfo() — prints in launcher banner

doc.go

@@ -0,0 +1,57 @@
+// 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