refactor: nest domain code under submitqueue/, extract shared layers

[behinddwalls]

Summary

Why?

SubmitQueue's domain code lived at the repo root (gateway/, orchestrator/,
entity/, extension/) while a second service family, Stovepipe, already lived
under stovepipe/. The layout was asymmetric and the line between
domain-specific and genuinely shared code was implicit. This restructure
gives every domain its own top-level folder and pulls shared code into
clearly labeled shared layers, so "what's shared vs domain" is obvious from
the directory tree.

What?

New layout — shared building blocks at the top level, one folder per domain:

core/             # SHARED cross-domain infra: errs, httpclient, metrics (no domain deps)
entity/           # SHARED entities: queue/
extension/        # SHARED extensions: queue/
submitqueue/      # SubmitQueue domain
├── gateway/
├── orchestrator/
├── entity/
├── extension/
└── core/         # consumer, request (shared between SubmitQueue's own services)
stovepipe/        # Stovepipe domain (gateway/ orchestrator/ entity/ extension/ core/)
example/
├── submitqueue/  # servers + clients + docker-compose
└── stovepipe/
test/
├── integration/{core,submitqueue,stovepipe}/
└── e2e/submitqueue/
doc/rfc/          # sql-queue-rfc.md (shared) + submitqueue/{workflow,build-runner}.md

• Domain code nests per domain: submitqueue/{gateway,orchestrator,entity,extension,core}, mirroring the existing stovepipe/ layout.
• Shared layers at the top level: core/ (cross-domain infra — errs, httpclient, metrics, now with zero domain deps), entity/ (shared entities: queue), extension/ (shared extensions: queue).
• submitqueue/core/ holds infra shared only between SubmitQueue's own services (consumer, request), moved out of the top-level core/ so it is domain-free. stovepipe/core/ added as a placeholder for symmetry.
• example/ and test/ mirror the split: example/submitqueue/..., test/integration/{core,submitqueue,stovepipe}/..., test/e2e/submitqueue/.
• RFCs: SubmitQueue-specific RFCs moved under doc/rfc/submitqueue/; the shared sql-queue RFC stays at doc/rfc/.
• Make targets made domain-explicit (build-submitqueue-, integration-test-submitqueue-, local-submitqueue-*), mirroring the existing -stovepipe- targets; CI updated to match.
• Tests: domain-qualified docker-compose project names; integration/e2e run in parallel (--test_output=errors) and always execute a real pass (tags=["external"]) since they are non-hermetic.
• Import paths rewritten throughout; protos regenerated; CLAUDE.md, READMEs, and doc/howto/TESTING.md updated to the new structure and naming convention.

No behavior changes — this is a structural move.

Test Plan

• ✅ make build (bazel build //...) and make test (40/40 unit tests)
• ✅ make integration-test — 8/8 pass; verified running in parallel (67s vs 275s serial) and always-run (uncached re-runs)
• ✅ make e2e-test — passes; always-run
• ✅ make lint (incl. license headers); gazelle / mocks / tidy / fmt all stable (CI gates clean)