feat(runway): dlq reconciler for merge topics

[behinddwalls]

Summary

Why?

SubmitQueue records in-flight merge work before publishing and then waits for exactly one MergeResult echoing its correlation id. Runway is stateless and the sole responder on that id, so every request must resolve to a result — or the client waits forever.

The primary controllers resolve what they can name: conflicts and invalid requests become a FAILED result, infrastructure faults are nacked for retry. But a fault that never recovers exhausts the retry budget and dead-letters. Nothing consumed those dead-letter topics, so the request produced no signal at all and the client's correlation id hung indefinitely.

What?

Adds runway/controller/dlq, a reconciler that subscribes to an inbound topic's _dlq queue and, for each dead-lettered MergeRequest, republishes a FAILED MergeResult echoing the correlation id to the corresponding signal topic. dlq.TopicKey derives the DLQ topic key from the primary one so the two stay in lockstep.

Unlike the SubmitQueue and Stovepipe DLQ reconcilers this one writes no entity state — Runway has none, and the signal is the resolution. A payload that cannot be decoded carries no correlation id and is dropped rather than retried forever.

Wires two instances in the server (one per inbound topic) on a dedicated consumer running under errs.AlwaysRetryableProcessor, so a transient publish failure retries indefinitely rather than dead-lettering the dead-letter. The DLQ consumer is started alongside the primary one and stopped with the same 30s drain on shutdown; both stop errors are joined into the exit status.

Test Plan

✅ bazel test //runway/... — 5/5 pass, including new //runway/controller/dlq coverage for republish-on-dead-letter, the drop-undecodable-payload path, and publish-failure propagation
✅ bazel build //service/runway/... — wiring compiles
✅ make gazelle, make fmt

Stack

1. @ feat(runway): dlq reconciler for merge topics #459
2. feat(runway): git-backed merger with REBASE #460
3. feat(runway): git merger SQUASH_REBASE and MERGE #461
4. feat(runway): git merger PROMOTE #462
5. feat(runway): wire the git merger into the server #463
6. feat(runway): reject changes that disagree on provider #476
7. docs: define the URI as the unit of change #477