(remote): head-turn refresh failure logging and IPC guard (follow-up #872) #873

Follow-up of #872 (#701), review minors.

1. Log volume. remote-index.js ~296 logs one warning per host on every refresh while the local cache keeps failing. Log only when the error message changes, or once until the next success.
2. Coverage of the main.js wrapper. test/head-turn-review.test.js ~237 still evaluates the main.js wrapper without { filename }, so c8 does not count it from there. Add { filename }.
3. IPC rejection. The get-task-snapshots handler (main.js ~3378) awaits refreshRemoteHeadTurns() unguarded, so a cache read error rejects the IPC call. This predates (remote): keep a host healthy when its head-turn refresh fails (#701) #872. Catch the error and log it, like the per-host path.