feat: share receipt validators

[intel352]

Summary

• add public proof, service, and edge receipt payload types to compute-core
• add credential-HMAC and software-fallback receipt signature helpers
• add service/edge receipt verifier helpers with software fallback denied by default

Verification

• GOWORK=off go test ./protocol -run 'Test(Proof|Service|Edge).*Receipt|TestCredential.*Signature|TestSoftware.*Signature' -count=1
• GOWORK=off go test ./... -count=1
• git diff --check

Part of workflow-compute slimming Phase 2B sub-slice; workflow-compute consumer PR follows after this merges.