feat(stovepipe): add new Ping-only service scaffold

[behinddwalls]

What

Adds a new stovepipe service alongside gateway and orchestrator, scaffolded end-to-end:

• stovepipe/proto/stovepipe.proto — SubmitQueueStovepipe.Ping RPC + protopb BUILD
• stovepipe/controller/ping.go (+ 5 unit tests) — logger + tally metrics + service_name: "stovepipe"
• example/server/stovepipe/ — gRPC server binary on :8083 (:8080 in container), graceful shutdown on SIGINT/SIGTERM, gRPC reflection
• example/server/stovepipe/{Dockerfile,docker-compose.yml} — single-container compose stack (no MySQL)
• example/client/stovepipe/ — arh-style ping client mirroring the gateway/orchestrator clients
• test/integration/stovepipe/ — Docker-based gRPC integration test
• Makefile — build-stovepipe-linux, integration-test-stovepipe, local-stovepipe-{start,stop}, run-client-stovepipe, plus stovepipe entries in proto, clean-proto, and build-all-linux
• BUILD.bazel — gazelle:resolve directive to disambiguate the stovepipe/protopb import path

Why

Lands the wiring now so future RPCs can be added without re-doing the proto/build/docker/test plumbing. Diverges from the gateway/orchestrator boilerplate in one place: stovepipe is stateless — no MySQL or queue — because Ping doesn't need them and carrying the boilerplate would be dead weight. The shape of main.go still matches gateway closely, so growing in DB/queue dependencies later is a small delta.

Test Plan

Local verification on this branch:

• make proto && make gazelle — clean, no further changes after directive
• make lint — passes (fmt + license headers)
• make check-tidy — go.mod / MODULE.bazel clean
• make check-gazelle — BUILD files in sync
• bazel test //stovepipe/controller:controller_test — 5/5 ping unit tests pass
• bazel build //stovepipe/... //example/server/stovepipe //example/client/stovepipe //test/integration/stovepipe:stovepipe_test — all targets build
• make integration-test-stovepipe — Docker build + gRPC Ping round-trip passes (~40s); server log confirms service_name: "stovepipe" response
• CI green on this PR

Manual smoke (optional):

make local-stovepipe-start
make run-client-stovepipe SERVER_ADDR=localhost:$(docker port submitqueue-stovepipe-service-1 8080 | cut -d: -f2) MESSAGE=hello
make local-stovepipe-stop

Out of scope

• No MySQL/queue/counter/storage wiring (stateless service).
• No changes to test/e2e/ — that suite tests the gateway→orchestrator SubmitQueue flow, which stovepipe isn't part of.
• No new business logic — PingController is the only controller.