Add golden test data for via exit route steering and fix via exit grant compilation to match Tailscale SaaS behavior. Includes MapResponse golden tests for via grant route steering verification.
Updates #2180
Add three tests that verify control plane behavior for grant policies:
- TestGrantViaSubnetFilterRules: verifies the router's PacketFilter
contains destination rules for via-steered subnets. Without per-node
filter compilation for via grants, these rules were missing and the
router would drop forwarded traffic.
- TestGrantViaExitNodeFilterRules: same verification for exit nodes
with via grants steering autogroup:internet traffic.
- TestGrantIPv6OnlyPrefixACL: verifies that address-based aliases
(Prefix, Host) resolve to exactly the literal prefix and do not
expand to include the matching node's other IP addresses. An
IPv6-only host definition produces only IPv6 filter rules.
Updates #2180
Add servertest grant policy control plane tests covering basic grants, via grants, and cap grants. Fix ReduceRoutes in State to apply route reduction to non-via routes first, then append via-included routes, preventing via grant routes from being incorrectly filtered.
Updates #2180
