engine: park the hot loop while every transport is down (ibx#399)

[userFRM]

Summary

• The hot loop busy-polls the farm, auth and historical sockets and never yields. While a session is up that is the design: a poll that finds a tick returns it with no syscall in the way, and core_id exists so the spin can own a core.
• With every transport down there is nothing to poll. Each pass runs the same three polls, gets nothing, and starts again — measured at roughly 925,000 iterations a second, which is one core pinned at 100% for the length of the disconnect.
• Parking 1 ms in that state, and only that state, leaves the connected path byte for byte as it was. Nothing waits on the loop meanwhile: reconnects are already scheduled on the ibx#218 backoff in seconds, and shutdown is read once per pass.

On the reported cause

The report puts the CPU on the reconnect scheduler retrying in a tight loop. The timestamps in its own log are two minutes apart:

09:48:11.537  CCP auto-reconnect skipped: no credentials
09:50:21.157  CCP auto-reconnect skipped: no credentials

That is the 60-second re-check in maybe_spawn_ccp_reconnect doing its job. The spin is the loop body itself, and it is there whether or not credentials are cached — a connected engine spins too, which is intended, and a disconnected one spins for nothing, which is not.

Closes #399.

Test plan

• cargo test --lib — 803 pass. The two config::expiry_tests failures are pre-existing on main and identical on a pristine checkout of 9367845.
• a_loop_with_every_transport_down_does_not_spin drives run() with all three transports marked down, shuts it down after 60 ms and asserts the iteration count. Counting iterations rather than CPU time is what separates a parked loop from a spinning one on any machine.
• Mutation: deleting the sleep takes that test from ~60 iterations to 55,554 in the same window, and it fails by name.
• Not covered: the laptop sleep/resume path the report describes. The guard is on the disconnected flags, so it does not depend on how the disconnect happened.