Address order: sequential, reverse or random, per subnet and pool

- Which never-used pool address a new client gets: sequential (the
  lowest, as before), reverse or random, in the settings, a subnet or
  a single pool (FIRST-LAST:ORDER). When no never-used address is left,
  the one free the longest, whatever the order; returning clients keep
  theirs.
- A subnet's pools are used in the order listed, or one at random
  weighted by its never-used addresses. Pools now keep the order they
  are given in (no more sorting by address).
- CLI --address-order and --pool-order; --add-pool of an existing range
  changes its order in place. Web UI: per-pool order and a move-up
  button, pool and address order on the subnet and settings pages.
- Kea's allocator: random is imported. Schema migration 5.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dhcpd/handle_test.go

@@ -499,3 +499,84 @@ func TestLastSeen(t *testing.T) {
 		t.Errorf("pinged %v", h.ping.pinged)
 	}
 }
+
+func TestAddressOrder(t *testing.T) {
+	ip := func(o *dhcp4.Packet) string { return o.YIAddr.String() }
+	twoPools := func(st *model.State) {
+		st.Subnets[1].Pools = []model.Pool{pool("10.0.20.10-10.0.20.19"), pool("10.0.20.100-10.0.20.109")}
+	}
+	// Sequential (the default): the lowest of the first pool listed.
+	h := newHarness(t, twoPools)
+	if got := ip(h.lease("eth0.20", mac(1))); got != "10.0.20.10" {
+		t.Errorf("sequential: %s", got)
+	}
+	// Never-used addresses first: one that came free isn't the next.
+	h.clock = h.clock.Add(2 * time.Hour)
+	if got := ip(h.lease("eth0.20", mac(2))); got != "10.0.20.11" {
+		t.Errorf("after an expiry: %s", got)
+	}
+	// But the client that had it gets it back, whatever the order.
+	if got := ip(h.lease("eth0.20", mac(1))); got != "10.0.20.10" {
+		t.Errorf("returning client: %s", got)
+	}
+
+	// Reverse from the settings.
+	h = newHarness(t, func(st *model.State) { twoPools(st); st.Settings.AddressOrder = model.OrderReverse })
+	if got := ip(h.lease("eth0.20", mac(1))); got != "10.0.20.19" {
+		t.Errorf("reverse: %s", got)
+	}
+	// The pools in the order listed; a pool's own order wins over the
+	// subnet's, which wins over the settings'.
+	h = newHarness(t, func(st *model.State) {
+		st.Subnets[1].Pools = []model.Pool{{Start: netip.MustParseAddr("10.0.20.100"), End: netip.MustParseAddr("10.0.20.109"), Order: model.OrderSequential}, pool("10.0.20.10-10.0.20.19")}
+		st.Subnets[1].AddressOrder = model.OrderReverse
+		st.Settings.AddressOrder = model.OrderRandom
+	})
+	if got := ip(h.lease("eth0.20", mac(1))); got != "10.0.20.100" {
+		t.Errorf("pool's own order: %s", got)
+	}
+	// Full first pool: the next one, in the subnet's order (reverse).
+	h = newHarness(t, func(st *model.State) {
+		st.Subnets[1].Pools = []model.Pool{pool("10.0.20.10-10.0.20.11"), pool("10.0.20.100-10.0.20.109")}
+		st.Subnets[1].AddressOrder = model.OrderReverse
+	})
+	got := []string{ip(h.lease("eth0.20", mac(1))), ip(h.lease("eth0.20", mac(2))), ip(h.lease("eth0.20", mac(3)))}
+	if strings.Join(got, " ") != "10.0.20.11 10.0.20.10 10.0.20.109" {
+		t.Errorf("pool by pool: %v", got)
+	}
+
+	// Random inside a pool: the n-th never-used address.
+	h = newHarness(t, func(st *model.State) { twoPools(st); st.Subnets[1].AddressOrder = model.OrderRandom })
+	h.s.randN = func(n int) int { return 3 }
+	if got := ip(h.lease("eth0.20", mac(1))); got != "10.0.20.13" {
+		t.Errorf("random: %s", got)
+	}
+	if got := ip(h.lease("eth0.20", mac(2))); got != "10.0.20.14" {
+		t.Errorf("random, skipping the used one: %s", got)
+	}
+	// Random pools, weighted by never-used addresses: 10 + 10, so 12 is
+	// in the second pool, which then uses its own order.
+	h = newHarness(t, func(st *model.State) { twoPools(st); st.Subnets[1].PoolOrder = model.PoolsRandom })
+	var asked []int
+	h.s.randN = func(n int) int { asked = append(asked, n); return 12 }
+	if got := ip(h.lease("eth0.20", mac(1))); got != "10.0.20.100" || asked[0] != 20 {
+		t.Errorf("random pools: %s %v", got, asked)
+	}
+	// Real randomness stays inside the pools and never repeats.
+	h = newHarness(t, func(st *model.State) {
+		twoPools(st)
+		st.Subnets[1].PoolOrder = model.PoolsRandom
+		st.Subnets[1].AddressOrder = model.OrderRandom
+	})
+	seen := map[string]bool{}
+	for i := byte(1); i <= 20; i++ {
+		a := ip(h.lease("eth0.20", mac(i)))
+		if seen[a] || !(strings.HasPrefix(a, "10.0.20.1") && len(a) >= 10) {
+			t.Fatalf("random address %s (have %v)", a, seen)
+		}
+		seen[a] = true
+	}
+	if out := h.discover("eth0.20", mac(21)); out != nil {
+		t.Errorf("a full subnet offered %s", out.Packet.YIAddr)
+	}
+}