kona-node-service: shared single-chain composition entry point

Refactor `RollupNode::start()`'s per-chain wiring into a reusable single-chain composition entry point used by both kona-node and lokahi, so the two wirings cannot drift (a hand-copied second composition is the failure mode). Gate: kona-node acceptance suite unchanged.

Depends on: #22527.

🤖 *Co-created with Claude (Fable 5)*

Start by reading RollupNode::start() and tracing the per-chain wiring in kona-node and lokahi. Check the dependency in #22527 before choosing the composition boundary, then run the kona-node acceptance suite; done means both services use one reusable single-chain entry point and the suite remains unchanged.

Written by the indexing model from the issue text.