policy: surface exit nodes via autogroup:internet (#3212)

compileFilterRules skipped autogroup:internet destinations to keep them
out of the wire-format PacketFilter, but those same compiled rules are
the source of pm.matchers — and Node.CanAccess relies on a matcher whose
DestsIsTheInternet covers the public internet to surface exit-node peers
to ACL sources. With the skip in place no such matcher existed, exit
nodes silently dropped out of the source's peer list, and the docs'
exit-node walkthrough stopped working: `tailscale exit-node list`
returned "no exit nodes found" and `tailscale set --exit-node=<ip>`
returned "no node found in netmap with IP".

Drop the compile-time skip so autogroup:internet flows through normal
matcher derivation, and teach ReduceFilterRules to keep the resulting
client packet-filter rule on exit-route advertisers — Tailscale SaaS
sends those rules to exit nodes so the kernel filter accepts traffic
forwarded by autogroup:internet sources.

Verified against a live tailnet on 2026-04-28 via tscap; the b17/b18
captures land under testdata/issue_3212/ as a regression guard. The
captures are isolated from testdata/routes_results/ because the broader
TestRoutesCompat machinery assumes a CIDR-prefix wire format that
differs from the IPSet-range form SaaS emits for autogroup:internet —
aligning that wire format is tracked separately.

Fixes #3212

hscontrol/policy/policyutil/reduce.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/juanfont/headscale/hscontrol/util"
+	"go4.org/netipx"
 	"tailscale.com/tailcfg"
 )
 
@@ -18,6 +19,7 @@ import (
 func ReduceFilterRules(node types.NodeView, rules []tailcfg.FilterRule) []tailcfg.FilterRule {
 	ret := []tailcfg.FilterRule{}
 	subnetRoutes := node.SubnetRoutes()
+	hasExitRoutes := node.IsExitNode()
 
 	for _, rule := range rules {
 		// Handle CapGrant rules separately — they use CapGrant[].Dsts
@@ -56,6 +58,14 @@ func ReduceFilterRules(node types.NodeView, rules []tailcfg.FilterRule) []tailcf
 			// AllowedIPs/routing.
 			if slices.ContainsFunc(subnetRoutes, expanded.OverlapsPrefix) {
 				dests = append(dests, dest)
+				continue
+			}
+
+			// Exit-route advertisers need rules targeting the
+			// public internet so the kernel filter accepts
+			// traffic forwarded by autogroup:internet sources.
+			if hasExitRoutes && ipSetSubsetOf(expanded, util.TheInternet()) {
+				dests = append(dests, dest)
 			}
 		}
 
@@ -71,6 +81,20 @@ func ReduceFilterRules(node types.NodeView, rules []tailcfg.FilterRule) []tailcf
 	return ret
 }
 
+func ipSetSubsetOf(candidate, container *netipx.IPSet) bool {
+	if candidate == nil || container == nil {
+		return false
+	}
+
+	for _, pref := range candidate.Prefixes() {
+		if !container.ContainsPrefix(pref) {
+			return false
+		}
+	}
+
+	return true
+}
+
 // reduceCapGrantRule filters a CapGrant rule to only include CapGrant
 // entries whose Dsts match the given node's IPs. When a broad prefix
 // (e.g. 100.64.0.0/10 from dst:*) contains a node's IP, it is