fix: align DWN subscription protocol

[LiranCohen]

Summary

• align the Go WebSocket client with production structured ProgressToken frames and object acknowledgements
• demultiplex replay events before subscribe confirmation, preserve handler-before-cursor ordering, repair cursor gaps, and classify terminal versus retryable failures
• authenticate owner/grant topology subscriptions, watch recipient delivery records separately, and coalesce replay invalidations behind the establishment barrier
• keep role-only topology readers on the existing polling fallback until the scalable observer stream lands

Validation

• go build ./...
• go vet ./...
• go test ./... -count=1 -race

All required build, vet, and race gates pass.

Closes #253