feat: share service ingress evidence validation

[intel352]

Summary

• add ServiceIngressEvidence and terminal status contracts to compute-core
• move ingress evidence shape validation helpers into the public protocol package
• cover completed/failed evidence and unsafe helper/header/path cases

Verification

• GOWORK=off go test ./protocol -run TestServiceIngressEvidenceValidation -count=1
• GOWORK=off go test ./... -count=1
• git diff --check