doc: Add coordinator-loop liveness to the adapter guide

[ggevay]

I analyzed how SQL-806 slipped through AI reviews. It turns out that guide-adapter.md says nothing about keeping the coordinator loop responsive. The specific gaps seem to have been:

1. My Claude dismissed a perf cost concern with "the read-only coordinator serves no user traffic".
2. Didn't think about how this would scale with catalogs that are more than an order of magnitude bigger than most current tests in CI.
3. The PR just moved existing work from the end of the biased select to the beginning. It's an easy thing to miss how liveness-critical such a change can be.
4. There seems to have been some confusion about exactly what MissedTickBehavior::Skip does. (both in Claude and in my own head)

This adds a short subsection to guide-adapter.md's "Architecture & Key Concepts" that should take care of the above.

(I'm also thinking to do a followup PR that would add some numbers: E.g., assume catalog sizes of 30k, 1000 clusters, how slow msgs are acceptable.)

(Also, we might want to create a Release Qualification test that builds up a very big catalog and does various things on it, e.g. a 0dt upgrade. The spec sheet is close to this, but doesn't do a 0dt upgrade.)

🤖 Generated with Claude Code