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/

README.md

@@ -0,0 +1,54 @@
+# mysql
+
+`database/sql`-backed MySQL client with launcher lifecycle and health check integration.
+
+## Install
+
+```
+go get code.nochebuena.dev/go/mysql
+```
+
+## Usage
+
+```go
+db := mysql.New(logger, cfg)
+lc.Append(db)
+r.Get("/health", health.NewHandler(logger, db))
+```
+
+## Unit of Work
+
+```go
+uow := mysql.NewUnitOfWork(logger, db)
+
+err := uow.Do(ctx, func(ctx context.Context) error {
+    exec := db.GetExecutor(ctx) // returns the active Tx
+    _, err := exec.ExecContext(ctx, "INSERT INTO orders ...")
+    return err
+})
+```
+
+## Error mapping
+
+```go
+if err := db.HandleError(err); err != nil { ... }
+```
+
+| MySQL error | xerrors code |
+|---|---|
+| 1062 (ER_DUP_ENTRY) | `ErrAlreadyExists` |
+| 1216/1217/1451/1452 (foreign key) | `ErrInvalidInput` |
+| `sql.ErrNoRows` | `ErrNotFound` |
+| anything else | `ErrInternal` |
+
+## Configuration
+
+| Env var | Default | Description |
+|---|---|---|
+| `MYSQL_HOST` | required | Database host |
+| `MYSQL_PORT` | `3306` | Database port |
+| `MYSQL_USER` | required | Username |
+| `MYSQL_PASSWORD` | required | Password |
+| `MYSQL_NAME` | required | Database name |
+| `MYSQL_MAX_CONNS` | `5` | Max open connections |
+| `MYSQL_MIN_CONNS` | `2` | Max idle connections |