feat(db-sqlite): initial implementation — database/sql SQLite with lifecycle and UnitOfWork (v1.0.0)

Introduces code.nochebuena.dev/einherjar/db-sqlite — the SQLite database starter
for the Einherjar framework. Absorbs the sqlite package from micro-lib using the
pure-Go modernc.org/sqlite driver (no CGO, cross-compilation compatible).

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, Ping, HandleError
- Component — lifecycle.Component + observability.Checkable + Provider
- UnitOfWork — Do(ctx, fn)

Implementation:
- New(logger, cfg) Component — pool not created until OnInit
- OnInit: sql.Open with DSN (Path + Pragmas); sets MaxOpenConns, MaxIdleConns
- OnStart: Ping with 5s timeout; logs "sqlite: connected"
- OnStop: db.Close(); logs "sqlite: 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: wraps db.BeginTx; wrapped in xerrors on error
- HealthCheck: delegates to Ping; Priority LevelDegraded (file DB; absence
  is non-fatal on a server restart — file always present if configured)
- NewUnitOfWork(logger, provider) UnitOfWork — acquires writeMu before Begin
  to prevent SQLITE_BUSY under concurrent goroutines (SQLite single-writer model)
- writeMu: extracted from *sqliteImpl via type assertion in NewUnitOfWork;
  gracefully skipped when provider is a mock (nil mu)
- HandleError: sql.ErrNoRows→ErrNotFound, unique/pk constraint→ErrAlreadyExists,
  foreign key→ErrInvalidInput, all others→ErrInternal

Config (EINHERJAR_SQLITE_* env vars):
  Path(required), MaxOpenConns(1), MaxIdleConns(1),
  Pragmas(?_journal=WAL&_timeout=5000&_fk=true)

- Component interface embeds observability.Identifiable; identifiable.go implements
  ModulePath and ModuleVersion via runtime/debug.ReadBuildInfo() — prints in launcher banner

doc.go

@@ -0,0 +1,69 @@
+// Package sqlite provides a pure-Go SQLite client with lifecycle management,
+// health checks, and unit-of-work transaction support for Einherjar applications.
+//
+// # Overview
+//
+// [New] returns a [Component] that manages a [database/sql] connection pool,
+// satisfies the [lifecycle.Component] lifecycle hooks (OnInit, OnStart, OnStop),
+// and implements [observability.Checkable] with critical priority.
+//
+// [NewUnitOfWork] wraps multiple repository operations in a single serialized
+// transaction via context injection — no transaction object is passed between
+// functions. Write transactions are serialized through an internal mutex to
+// prevent SQLITE_BUSY errors under concurrent goroutines.
+//
+// # Lifecycle Registration
+//
+//	db := sqlite.New(logger, cfg)
+//	launcher.Register(db)
+//	health.Register(db)
+//
+// # Querying
+//
+// Repository code receives a [Provider] or [Executor] and calls [Provider.GetExecutor]
+// to obtain the active transaction (if inside a [UnitOfWork]) or the connection pool:
+//
+//	exec := db.GetExecutor(ctx)
+//	row := exec.QueryRowContext(ctx, "SELECT id FROM users WHERE email = ?", email)
+//	if err := row.Scan(&id); err != nil {
+//	    return db.HandleError(err)  // maps sql.ErrNoRows → ErrNotFound, etc.
+//	}
+//
+// # Unit of Work
+//
+// [NewUnitOfWork] wraps operations in a single serialized write transaction.
+// The transaction is injected into the context; [Provider.GetExecutor] returns
+// it automatically.
+//
+//	uow := sqlite.NewUnitOfWork(logger, db)
+//	err := uow.Do(ctx, func(ctx context.Context) error {
+//	    exec := db.GetExecutor(ctx) // returns active Tx
+//	    _, err := exec.ExecContext(ctx, "INSERT INTO orders ...")
+//	    return err
+//	})
+//
+// # Error Handling
+//
+// [HandleError] translates database/sql and SQLite error codes into typed
+// [xerrors] values. Call it at every point where a database error is first observed:
+//
+//	if err := row.Scan(&out); err != nil {
+//	    return db.HandleError(err)
+//	}
+//
+// Mapped codes:
+//   - sql.ErrNoRows                   → ErrNotFound
+//   - SQLITE_CONSTRAINT_UNIQUE (2067) → ErrAlreadyExists
+//   - SQLITE_CONSTRAINT_PRIMARYKEY (1555) → ErrAlreadyExists
+//   - SQLITE_CONSTRAINT_FOREIGNKEY (787)  → ErrInvalidInput
+//   - all others                      → ErrInternal
+//
+// # Configuration
+//
+// All fields are read from environment variables with the EINHERJAR_SQLITE_* prefix:
+//
+//   - EINHERJAR_SQLITE_PATH          (required) — file path or ":memory:"
+//   - EINHERJAR_SQLITE_MAX_OPEN_CONNS           — default: 1
+//   - EINHERJAR_SQLITE_MAX_IDLE_CONNS           — default: 1
+//   - EINHERJAR_SQLITE_PRAGMAS                  — default: "?_journal=WAL&_timeout=5000&_fk=true"
+package sqlite