feat(stovepipe): add BuildRequest/BuildSignal queue contract

[roychying]

Why?

Stovepipe's pipeline needs two more internal queue stages to carry a request through building and polling its build result: build (process/analyze hands off a request once its build scope is decided) and buildsignal (build hands off the triggered build so its status can be polled to a terminal state). Both stages' wire payloads need to be defined before their controllers can be implemented, per the algorithms in doc/rfc/stovepipe/steps/build.md and buildsignal.md.

What?

• Added buildsignal.proto / generated buildsignal.pb.go, defining BuildSignal { id } — the build id to poll, following the same "id only" convention as the rest of the internal pipeline (producer and consumer share the store, so redelivery stays idempotent).
• Added TopicKeyBuildSignal alongside the existing TopicKeyBuild in topics.go, and the BuildSignal alias in messagequeue.go.
• • Renamed process_test.go → messagequeue_test.go since its round-trip and topic-key-binding guard tests are package-wide, not process-specific, and extended them to cover BuildSignal (round-trip test, wire-format check, and the TestTopicKeysBindEveryTopicKey drift guard).

Test Plan

• make build and make test - all clean

Issue