A cell child inside a preview reaches its parent for heartbeats, and the real child forge test fails rather than skips where it can run #6030

Follow-up from #6027 (review by @juniper).

1. A child forge in its own cell, spawned by a forge that is itself a preview in a cell, cannot reach its parent's URL, so its family heartbeat never lands and /forges shows it as silent. The child needs a path to the parent from inside its cell, such as a socket relayed into the cell the way in.sock is, so heartbeats arrive. Done when a test boots a cell child and sees its heartbeat recorded by the parent.

1. test/spawn-cell.test.ts > "a real child forge" prints SKIPPING: when the host refuses a cell inside the test. Once the check log shows the check host runs it, turn that skip into a failure there (for example, gate it on an env the check sets), so a regression cannot pass as a skip.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.