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

errors.go

@@ -0,0 +1,46 @@
+package sqlite
+
+import (
+	"database/sql"
+	"errors"
+
+	"code.nochebuena.dev/einherjar/core/xerrors"
+)
+
+// coder is the duck-type interface for SQLite extended error codes.
+// modernc.org/sqlite errors implement this interface.
+type coder interface{ Code() int }
+
+const (
+	sqliteConstraintPrimaryKey = 1555
+	sqliteConstraintUnique     = 2067
+	sqliteConstraintForeignKey = 787
+)
+
+// HandleError maps database/sql and SQLite errors to typed [xerrors] values.
+// Returns nil when err is nil. Also available as [Provider.HandleError].
+//
+// Mapped codes:
+//   - sql.ErrNoRows                      → ErrNotFound
+//   - SQLITE_CONSTRAINT_UNIQUE (2067)    → ErrAlreadyExists
+//   - SQLITE_CONSTRAINT_PRIMARYKEY (1555)→ ErrAlreadyExists
+//   - SQLITE_CONSTRAINT_FOREIGNKEY (787) → ErrInvalidInput
+//   - all others                         → ErrInternal
+func HandleError(err error) error {
+	if err == nil {
+		return nil
+	}
+	if errors.Is(err, sql.ErrNoRows) {
+		return xerrors.New(xerrors.ErrNotFound, "record not found").WithError(err)
+	}
+	var ce coder
+	if errors.As(err, &ce) {
+		switch ce.Code() {
+		case sqliteConstraintUnique, sqliteConstraintPrimaryKey:
+			return xerrors.New(xerrors.ErrAlreadyExists, "record already exists").WithError(err)
+		case sqliteConstraintForeignKey:
+			return xerrors.New(xerrors.ErrInvalidInput, "data integrity violation").WithError(err)
+		}
+	}
+	return xerrors.New(xerrors.ErrInternal, "unexpected database error").WithError(err)
+}