A grant of the form `{src: alice, dst: autogroup:internet, via:
tag:exit1}` was loading without error but stripping every exit node
from alice's view: `tailscale exit-node list` returned "no exit nodes
found".
Two sites skipped autogroup:internet at the compile / steering layer:
compileViaForNode's *AutoGroup arm produced no FilterRule for the
via-tagged exit node, and ViaRoutesForPeer's *AutoGroup arm produced
no Include/Exclude. With pm.needsPerNodeFilter true, the exit node's
matchers were empty, BuildPeerMap could not link source to exit, and
RoutesForPeer's ReduceRoutes stripped 0.0.0.0/0 and ::/0 from
AllowedIPs.
The skip belongs at the wire-format layer (ReduceFilterRules), not at
the compile layer that also feeds internal matchers. Lift
autogroup:internet handling into both *AutoGroup arms with the same
shape used for *Prefix destinations: emit a TheInternet rule on
via-tagged exit advertisers; surface peer.ExitRoutes() in Include
when the peer carries the via tag, Exclude otherwise.
ReduceFilterRules continues to keep the rule on exit-route
advertisers' wire output and strip it elsewhere, preserving SaaS
PacketFilter encoding.
Also drop compileViaForNode's early len(SubnetRoutes)==0 return:
SubnetRoutes excludes exit routes, so the early return pre-empted the
autogroup:internet branch on nodes that only advertise exit routes.
Existing tests pinning the buggy behaviour (TestViaRoutesForPeer
subtests, TestCompileViaGrant case) flipped to the new contract.
Fixes#3233
2b7f15abaa