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/

go.sum

@@ -0,0 +1,12 @@
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+github.com/go-sql-driver/mysql v1.8.1/go.mod h1:wEBSXgmK//2ZFJyE+qWnIsVGmvmEKlqwuVSjsCm7DZg=