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

README.md

@@ -0,0 +1,139 @@
+# einherjar/db-mysql
+
+[![version](https://img.shields.io/badge/version-v1.0.0-5C4EE5?style=flat-square)](https://code.nochebuena.dev/einherjar/db-mysql)
+[![license](https://img.shields.io/badge/license-AGPL--3.0-22863A?style=flat-square)](LICENSE)
+[![go](https://img.shields.io/badge/Go-1.26+-00ADD8?style=flat-square&logo=go&logoColor=white)](https://go.dev)
+[![health](https://img.shields.io/badge/health-critical-D73A49?style=flat-square)]()
+
+> A warrior's deeds are committed to stone so that what they built survives the builder.
+
+`code.nochebuena.dev/einherjar/db-mysql` is the MySQL/MariaDB database component of the Einherjar framework. It wraps `database/sql` with `go-sql-driver/mysql` behind a lifecycle-aware `Component`, exposes a uniform `Executor` interface for queries and transactions, and provides a `UnitOfWork` that injects the active transaction into context — so repositories never need to know whether they are inside a transaction or not.
+
+---
+
+## Usage
+
+### Setup
+
+```go
+import dbmysql "code.nochebuena.dev/einherjar/db-mysql"
+
+db := dbmysql.New(logger, dbmysql.DefaultConfig())
+launcher.Append(db)   // OnInit opens pool; OnStop closes it
+health.Register(db)   // PING health check; LevelCritical
+```
+
+### Querying
+
+```go
+// GetExecutor returns the pool when called outside a UnitOfWork.
+rows, err := db.GetExecutor(ctx).QueryContext(ctx, "SELECT id, name FROM users WHERE active = ?", true)
+defer rows.Close()
+
+var name string
+err := db.GetExecutor(ctx).QueryRowContext(ctx, "SELECT name FROM users WHERE id = ?", id).Scan(&name)
+```
+
+### Manual transaction
+
+```go
+tx, err := db.Begin(ctx)
+if err != nil {
+    return err
+}
+defer tx.Rollback()
+
+_, err = tx.ExecContext(ctx, "UPDATE accounts SET balance = balance - ? WHERE id = ?", amount, fromID)
+if err != nil {
+    return err
+}
+return tx.Commit()
+```
+
+`BeginTx` is available when you need explicit isolation level control:
+
+```go
+tx, err := db.BeginTx(ctx, &sql.TxOptions{Isolation: sql.LevelSerializable})
+```
+
+### Unit of work (recommended)
+
+`UnitOfWork` wraps the transaction in context so every call to `GetExecutor` inside `uow.Do` automatically returns the active transaction. Repositories require no changes.
+
+```go
+uow := dbmysql.NewUnitOfWork(logger, db)
+
+err := uow.Do(ctx, func(ctx context.Context) error {
+    _, err := db.GetExecutor(ctx).ExecContext(ctx, "INSERT INTO orders (...) VALUES (...)", ...)
+    return err
+})
+```
+
+If the function returns an error, the transaction is rolled back. If it returns nil, the transaction is committed.
+
+### Error handling
+
+```go
+if err := db.HandleError(someErr); err != nil {
+    // MySQL error numbers mapped to xerrors:
+    // 1062 ER_DUP_ENTRY      → ErrAlreadyExists
+    // 1216 ER_NO_REFERENCED_ROW → ErrPreconditionFailed
+    // 1048 ER_BAD_NULL_ERROR → ErrInvalidInput
+    // context.Canceled       → ErrCancelled
+    // context.DeadlineExceeded → ErrDeadlineExceeded
+}
+```
+
+`HandleError` is also available as a package-level function: `dbmysql.HandleError(err)`.
+
+---
+
+## Environment variables
+
+| Variable | Required | Default | Description |
+|---|---|---|---|
+| `EINHERJAR_MYSQL_HOST` | Yes | — | MySQL host |
+| `EINHERJAR_MYSQL_PORT` | No | `3306` | Listen port |
+| `EINHERJAR_MYSQL_USER` | Yes | — | Database user |
+| `EINHERJAR_MYSQL_PASSWORD` | Yes | — | Database password |
+| `EINHERJAR_MYSQL_NAME` | Yes | — | Database name |
+| `EINHERJAR_MYSQL_MAX_CONNS` | No | `5` | Maximum open connections |
+| `EINHERJAR_MYSQL_MIN_CONNS` | No | `2` | Minimum idle connections |
+| `EINHERJAR_MYSQL_MAX_CONN_LIFETIME` | No | `1h` | Maximum connection lifetime |
+| `EINHERJAR_MYSQL_MAX_CONN_IDLE_TIME` | No | `30m` | Maximum idle time before connection is closed |
+| `EINHERJAR_MYSQL_CHARSET` | No | `utf8mb4` | Connection charset |
+| `EINHERJAR_MYSQL_LOC` | No | `UTC` | Timezone location |
+| `EINHERJAR_MYSQL_PARSE_TIME` | No | `true` | Parse `DATE`/`DATETIME` as `time.Time` |
+
+> **Note:** Set collation at the schema level (DDL), not in the DSN. MariaDB 11.4+ collation names exceed the 1-byte handshake limit in `go-sql-driver`.
+
+---
+
+## Dependency graph
+
+```
+contracts  (zero dependencies)
+    ↑
+  core
+    ↑
+db-mysql  (contracts, core, go-sql-driver/mysql)
+    ↑
+  your app
+```
+
+---
+
+## Verification
+
+```bash
+cd db-mysql/
+go build ./...
+go vet ./...
+go test ./...
+gofmt -l .
+```
+
+---
+
+> *The hall is built before the warriors arrive.*
+> *That is the only guarantee worth making.*