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

unit_of_work.go

@@ -0,0 +1,17 @@
+package sqlite
+
+import "context"
+
+// UnitOfWork wraps a set of repository operations in a single serialized
+// database transaction. The transaction is injected into the context;
+// [Provider.GetExecutor] returns it automatically so repository code requires
+// no changes to participate.
+//
+// Write transactions are serialized through a mutex to prevent SQLITE_BUSY
+// errors. Pass the result of [New] to [NewUnitOfWork] to enable serialization.
+type UnitOfWork interface {
+	// Do begins a transaction, calls fn with the enriched context, and commits
+	// on success or rolls back on error. The original fn error is always returned
+	// when fn fails, regardless of the rollback outcome.
+	Do(ctx context.Context, fn func(ctx context.Context) error) error
+}