feat: apply provider conformance evidence

[intel352]

Summary

• adds ProviderContract.ApplyProviderConformanceEvidence to compute-core protocol
• centralizes tuple validation and runtime profile promotion to real-client conformance
• keeps conformance profile application idempotent

TDD

• red: TestProviderContractAppliesProviderConformanceEvidence failed because ProviderContract had no ApplyProviderConformanceEvidence method
• green: implemented the protocol method and idempotent conformance profile append

Verification

• GOWORK=off go test ./protocol -run 'TestProviderContractAppliesProviderConformanceEvidence|TestProviderConformanceEvidenceRequiresArtifactDigestAndObservation|TestProviderRuntimeProfileRequiresRealClientEvidence' -count=1
• GOWORK=off go test ./... -race -count=1
• git diff --check