feat: implement ZeroTier exit gateway switcher

Adds the luci-app-zt-gateway package: a LuCI app + rpcd/ucode backend +
shell switch script that reconfigures which remote ZeroTier node acts as
the internet exit gateway for WIBLAN clients (10.11.13.0/24).

  - Makefile (luci.mk, arch-independent)
  - UCI config skeleton with three sample gateways
  - rpcd ACL + menu entry
  - zt-gateway.uc rpcd backend exposing status / switch / health /
    drain_status / cancel_drain ubus methods
  - zt-gateway-switch shell script implementing force + graceful modes:
      * force: pre-flight ping, atomic route replace, conntrack flush,
        UCI/hotplug/rc.local persistence
      * graceful: dual-table drain using CONNMARK fwmark 0x100 at
        priority 99, background drain monitor with two-consecutive-zero
        completion and timeout-forced fallback to force
  - LuCI overview.js: gateway radio list, mode select, drain progress
    panel, cancel-drain button, health polling
  - Docker test harness (docker-compose + Dockerfile.router +
    router/gw entrypoints) exercising the switch script against real
    iproute2/iptables/conntrack on two simulated exit nodes

Verified against the harness: force switch, graceful drain to natural
completion, pre-flight blocking of unreachable gateways (force + graceful),
and drain-timeout forced fallback.

root/usr/share/rpcd/acl.d/luci-app-zt-gateway.json

@@ -0,0 +1,25 @@
+{
+	"luci-app-zt-gateway": {
+		"description": "ZeroTier Exit Gateway switching",
+		"read": {
+			"uci": ["zt-gateway", "network"],
+			"ubus": {
+				"zt-gateway": ["status", "health", "drain_status"]
+			},
+			"file": {
+				"/etc/hotplug.d/net/99-zerotier-bridge": ["read"],
+				"/etc/rc.local": ["read"]
+			}
+		},
+		"write": {
+			"uci": ["zt-gateway", "network"],
+			"ubus": {
+				"zt-gateway": ["switch", "cancel_drain"]
+			},
+			"file": {
+				"/etc/hotplug.d/net/99-zerotier-bridge": ["write"],
+				"/etc/rc.local": ["write"]
+			}
+		}
+	}
+}