fix(submitqueue): keep internal merge stage topic distinct from runway merge queue

[sbalabanov]

Summary

The orchestrator registers its internal merge pipeline stage and the runway-owned merge queue in one topic registry over one shared queue backend, and both used the topic key "merge" (topickey.TopicKeyMerge == runwaymq.TopicKeyMerge). The publish-only runway entry silently shadowed the internal stage entry (last write wins on the registry maps), collapsing the two topics into one:

• runway consumed the internal batch-ID messages as zero-step MergeRequests and reported SUCCEEDED without ever seeing the real steps;
• the orchestrator merge controller's real MergeRequest publish was then silently dropped by the queue's (topic, partition_key, id) dedupe, because it reuses the batch ID already published to the same topic.

The pipeline still reached landed only because the zero-step result correlates by batch ID — with a real merger, nothing would ever have been merged. Found by the new e2e harness (stacked PR) asserting that runway's merge signal carries the request's step IDs.

Changes

• submitqueue/core/topickey: rename the internal merge stage key (and its orchestrator wire name) to mergebatch, with a comment explaining why it must not be "merge".
• platform/consumer: NewTopicRegistry now rejects duplicate topic keys, so a future collision fails at startup instead of silently misrouting messages.
• service/submitqueue/orchestrator: register the internal stage under the new wire name.

Test plan

• bazel test //... --test_tag_filters=-manual,-integration — 75/75 pass (includes the updated dlq controller test).
• bazel test //test/integration/... — 8/8 pass.
• e2e suite (with the stacked harness PR): runway's merge signal now carries the request step IDs and the full land pipeline completes in ~2s.

🤖 Generated with Claude Code

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[behinddwalls]

should we namespace these to say runway-merge vs just merge and same from SQ stuff to be submitqueue-merge in keys?