pool: add keyed worker messages

[raphael]

Summary

• Add DispatchMessage for hash-ring-routed worker messages that do not create or depend on job ownership.
• Keep NotifyWorker job-scoped while introducing an explicit MessageHandler contract for message-capable workers.
• Document message semantics and cover routing, missing-handler errors, and keyed payload encoding with focused tests.

Test plan

• GOWORK=off go test ./pool -run 'TestDispatchMessage|TestNotifyWorkerNoHandler|TestMarshalKeyedPayload' -count=1
• GOWORK=off go test -p 1 ./... -count=1

Reviewer notes

Start with pool/node.go and pool/worker.go; the rest is codec reuse, tests, and README contract documentation.