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

go.sum

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+filippo.io/edwards25519 v1.1.0 h1:FNf4tywRC1HmFuKW5xopWpigGjJKiJSV0Cqo0cJWDaA=
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+github.com/go-sql-driver/mysql v1.8.1 h1:LedoTUt/eveggdHS9qUFC1EFSa8bU2+1pZjSRpvNJ1Y=
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