docs(worker): correct tier from 2 to 3 and fix dependency tier refs

worker depends on launcher (now correctly Tier 2) and logz (Tier 1),
placing it at Tier 3. The previous docs cited launcher as Tier 1 and
logz as Tier 0, both of which were wrong.

doc.go

@@ -0,0 +1,8 @@
+// Package worker provides a concurrent worker pool with launcher lifecycle integration.
+//
+// Usage:
+//
+//	pool := worker.New(logger, worker.Config{PoolSize: 5, BufferSize: 100})
+//	lc.Append(pool)
+//	pool.Dispatch(func(ctx context.Context) error { return doWork(ctx) })
+package worker