autocert's HTTPHandler answers 404 itself for a token it does not hold
and never consults its fallback on the challenge path — the
predecessor's exact fault, rediscovered live when Mailu's renewal died
behind this proxy on cutover day. tokenOrRoute probes each authority
against a buffered writer and hands a token none of them holds to plain
routing, so a consumer's own ACME client answers its own challenge
through an ordinary path-scoped route. Four tests pin it, including the
cache-key shape a restart-surviving token actually has.
jschoubben