refactor: split flat layout into routes/, domain/, infra/

[AbstractNucleus]

What: discovery.py, health.py, membership.py, tombstones.py, migration.py, scaffolding/ all live flat next to routes/.

Why: The mental model "discovery → DB → routes → docker_client" is implicit. The codebase will get harder to navigate as it grows.

Approach: Reshape into mcontrol/{routes,domain,infra}/. infra/ holds db, docker_client, rcon; domain/ holds discovery, lifecycle_state, membership, tombstones. Pure mechanical move — no behaviour change.

[AbstractNucleus]

This was generated by AI during triage.

Routed to ready-for-human — pure design preference on how to slice the package. The mechanical move is straightforward but the grouping (what counts as domain/ vs infra/) wants a human eye, especially around the boundary cases (scaffolding/, migration.py, health.py).

Easy to lift to ready-for-agent once the target layout is sketched.

[AbstractNucleus]

Triage note: this is a high-conflict refactor (it touches imports across most of the codebase). Recommendation is to land it after all other open feature bundles to avoid merge-conflict churn — specifically after #119, #123, and the remaining a11y surface slices of #110. Otherwise every in-flight branch has to be rebased through this rename.

Same recommendation applies to #97 and #101.

Holding off until the queue drains; if priority changes, ping here and I'll pick it up.