M6: anycast — pod lo + Ready-gated /128/32 + BIRD export

CNI ADD now adds anycast IPs to the pod's lo interface (NOT eth0 — design
doc rationale: avoid NDP/ARP DAD conflicts when N replicas share an IP).
Allocation persists the anycast list.

AnycastReconciler:
  desired = { ip → flock<8hex> } from
            committed allocations × pod.Status.PodReady=True
  diff against advertised, install/remove host /128 (v6) or /32 (v4)
  re-render bird.conf with the active set

Triggers: 2s tick, AfterCommit (per ADD/DEL), Pod informer Ready
transitions (PodCache.OnReadyChange callback).

The bird template already supported Anycast6/Anycast4 via the export
filter — this turn finally drives those slices from runtime.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

pkg/agent/netns_stub.go

@@ -15,6 +15,7 @@ type SetupRequest struct {
 	HostIface   string
 	IP6         net.IP
 	IP4         net.IP
+	Anycast     []net.IP
 }
 
 // Setup is unimplemented on non-Linux platforms; the agent only runs in