Return the repository-standard not-found response when an upstream module is absent.
Constraint: GOPROXY advances to direct only after 404 or 410 responses.
Rejected: Preserve a handler-specific response body | sibling handlers consistently use not found.
Confidence: high
Scope-risk: narrow
Directive: Keep missing-artifact responses consistent across registry handlers.
Tested: go test ./internal/handler -run TestGoModuleDownloadUpstreamErrors -count=1; prior go test -race ./...; go build ./...; golangci-lint; go vet.