feat(mysql): initial stable release v0.9.0

database/sql-backed MySQL client with launcher lifecycle, health check, unit-of-work via context injection, and structured error mapping.

What's included:
- Executor / Tx / Client / Component interfaces using database/sql native types (sql.Result, *sql.Rows, *sql.Row)
- Tx.Commit() / Tx.Rollback() without ctx, matching the honest database/sql contract
- New(logger, cfg) constructor; *sql.DB opened in OnInit
- Config struct with env-tag support for all pool tuning parameters
- UnitOfWork via context injection; GetExecutor(ctx) returns active *sql.Tx or *sql.DB
- HandleError mapping MySQLError.Number to xerrors codes (1062 → AlreadyExists, 1216/1217/1451/1452 → InvalidInput, ErrNoRows → NotFound)
- Driver imported as mysqldrv alias to avoid package name collision
- health.Checkable at LevelCritical; HealthCheck delegates to db.PingContext

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

errors.go

@@ -0,0 +1,31 @@
+package mysql
+
+import (
+	"database/sql"
+	"errors"
+
+	mysqldrv "github.com/go-sql-driver/mysql"
+
+	"code.nochebuena.dev/go/xerrors"
+)
+
+// HandleError maps MySQL and database/sql errors to xerrors types.
+// Also available as client.HandleError(err).
+func HandleError(err error) error {
+	if err == nil {
+		return nil
+	}
+	var mysqlErr *mysqldrv.MySQLError
+	if errors.As(err, &mysqlErr) {
+		switch mysqlErr.Number {
+		case 1062: // ER_DUP_ENTRY
+			return xerrors.New(xerrors.ErrAlreadyExists, "record already exists").WithError(err)
+		case 1216, 1217, 1451, 1452: // foreign key violations
+			return xerrors.New(xerrors.ErrInvalidInput, "data integrity violation").WithError(err)
+		}
+	}
+	if errors.Is(err, sql.ErrNoRows) {
+		return xerrors.New(xerrors.ErrNotFound, "record not found").WithError(err)
+	}
+	return xerrors.New(xerrors.ErrInternal, "unexpected database error").WithError(err)
+}