feat: add orchestrator pipeline, TopicRegistry, and CI improvements

[behinddwalls]

Summary

• TopicRegistry: Centralizes queue topic configuration, decoupling controllers from subscription details. Controllers declare their topic and consumer group; the registry provides the queue and subscription config.
• Orchestrator pipeline: Adds request controller with publisher for forwarding messages to the next pipeline stage (request → batch → speculation → build → merge).
• Consumer refactor: Accepts TopicRegistry instead of raw queue.Queue + subscriber name. Removes SubscriptionConfig() from the Controller interface.
• Server startup simplification: Removes retry loops from gateway and orchestrator servers — relies on Docker Compose healthchecks (--wait) to ensure dependencies are ready before services start.
• CI caching fix: Caches bazel-disk-cache and bazel-repo-cache (matching .bazelrc paths) with per-job keys to avoid concurrent save races. Previous setup cached the wrong directory and all jobs raced on a single key.
• Auto-tail container logs: Streams Docker container logs to stderr in real-time during integration/e2e tests via ComposeStack.tailLogs(). Removes the now-redundant .github/actions/logs action.
• Test output streaming: All test targets use --test_output=streamed for real-time output in both local and CI runs.
• Documentation consolidation: Merges PROJECT_STRUCTURE.md into CLAUDE.md, updates TESTING.md for new ComposeStack behavior.

Test plan

• make build passes
• make test passes
• make gazelle produces no changes
• CI integration and e2e tests pass
• Verify container logs stream to stderr during test runs
• Verify CI Bazel cache saves per-job and restores on subsequent runs
• First Run: https://github.com/uber/submitqueue/actions/runs/22336834292/job/64632139444?pr=59 (~10m)
• Second Run: https://github.com/uber/submitqueue/actions/runs/22337340518/job/64633633575?pr=59 (1m24s)
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