Salvage of #71282 (Fixes#71281): a routable-but-dead endpoint (corp
LAN address while off-VPN) blackholes TCP SYNs, so every probe in the
model-metadata waterfall waits out its full connect timeout — 20+
seconds of stall per startup across detect_local_server_type,
fetch_endpoint_model_metadata, and the per-model probes.
A module-level blackhole cache keyed on host:port is populated when
any probe observes a ConnectTimeout (httpx or requests; read timeouts
deliberately excluded — an accepted connection is not a blackhole) and
consulted at the top of each guarded function. 30s TTL: long enough to
collapse one startup burst, short enough that VPN recovery is picked
up without a restart. Guard ordering: blackhole check -> disk L2 ->
HTTP waterfall, and a blackholed leg aborts the remaining legs instead
of letting each stall in turn.
Squash of the PR's two real commits (the branch's merge commits made
it un-rebase-merge-able; content verified identical via merge-tree).
rlaope