applyTransport's switch only covered tcp/ws/grpc; xhttp and
httpupgrade inbounds fell through to the default branch and returned
false. buildProxy then returned a nil map and the inbound was dropped
from the Clash subscription. When the subscription only contained
xhttp/httpupgrade inbounds, the proxies list ended up empty and the
client saw a 404 (or an "Error!" body on older builds), then refused
to parse.
Add a case for each, mapping the inbound's stream settings onto the
Mihomo-format opts blocks:
xhttp -> xhttp-opts: { path, host, mode }
httpupgrade -> http-upgrade-opts: { path, headers: { Host } }
Host falls back to the headers map when the dedicated `host` field is
empty, matching the existing ws behavior.
Closes#4531
