[Test] Routing recipes bound by no test: the self-healing `Router.roundRobin(n, BackoffSupervisor.factory(…))` pool, and `smallest-mailbox` over the worker mesh

[pathosDev]

Two recipes the #170 routing-docs sweep recommends are exercised by no test in the tree (2026-09-18 verification):

1. The self-healing pool — Router.roundRobin(4, BackoffSupervisor.factory({ child: Worker })) — is recommended in four places (routing/router.mdx:191-200, routing/overview.mdx:158-166, both DE mirrors). It works by construction (verified by hand: child-1 self-stops, the supervisor logs cause: "stop", respawns child-2, both routees answer again, nothing pruned), but grep -rl BackoffSupervisor tests/ | xargs grep -l 'Router\.' returns nothing. One integration test that stops a child and asserts the pool's size and answers.
2. smallest-mailbox over the MESH transport. tests/unit/parallelism/ClusterRouterOverMesh.test.ts:150-153 asserts only the precondition (_isServing on every hosted worker); no test spawns a smallest-mailbox ClusterRouter over the mesh and observes a worker's depth reading arrive. The round trip is bound over another transport (tests/integration/in-process/cluster/router/ClusterRouterSmallestMailbox.test.ts:82,167); the recipe's own transport is not.

Refs #170.

Filed from the 2026-09-18 issue wave (#170 verification (concerns 2, 5)).