slices.Contains required exact equality between grant dst and the
advertised subnet route. Any non-identical pair was rejected, so a
via grant with broader (or narrower) dst emitted no filter rule and
added no route to the viewer's AllowedIPs. Tailscale SaaS uses
containment in either direction.
Switch to slices.ContainsFunc(routes, dst.Overlaps) for filter rule
emission (keep dst literal in DstPorts), and append overlapping
advertised routes to ViaRoutesForPeer.Include / Exclude. Rewrite the
multi-router HA election and regular-grant overlap detection to key
off the matched routes rather than the dst. Resolve *Host aliases to
*Prefix once in compileOneViaGrant and at the top of ViaRoutesForPeer
so the switch arms reach them.
Fixes#3267
e5fcd01ee6