v0.96.0 — R-50 island NIC: provision attaches the guest island net1

- LocalAPIConfig.island_bridge + island_guest_addr (+ IslandEnabled, Validate
  all-or-nothing + CIDR guard)
- buildBringUpConfig attaches static net1 (island) on provision + DR when set;
  absent otherwise (pre-R-50 byte-for-byte). Plumbed from cfg.LocalAPI at both
  RunBringUp sites. Endpoint already follows listen_addr (A0: no template change).
- healer stays eth0-only (A3 verify-only) — red-proof test locks the scoping
- example config + firewall example rewritten for the island; REUSE updated
- 3 non-hollow tests; full green. MinAgent unchanged.

Coupling: host-install island config requires agent >= 0.96.0 (vouch first).

internal/guestnet/watchdog_test.go

@@ -548,3 +548,21 @@ func TestClassify_Table(t *testing.T) {
 }
 
 var _ io.Writer = (*bytes.Buffer)(nil)
+
+// A3 (R-50): the guestnet healer is eth0-only and MUST stay blind to the island NIC. A guest on an
+// island host presents eth0 DHCP (the LAN leg the healer owns) PLUS eth1 static (the island). Because
+// parseMode is interface-scoped, adding eth1 static cannot flip eth0's detected mode — so the healer
+// keeps treating eth0 as DHCP and never runs dhclient against the static island NIC (which would
+// sabotage it). This is the verify-only guarantee that let R-50 ship the island NIC without a healer
+// change. Red-proof: make parseMode scan globally instead of per-dev and the eth0 assertion fails.
+func TestParseMode_IslandStaticNICDoesNotConfuseEth0(t *testing.T) {
+	interfaces := "auto lo\niface lo inet loopback\n\n" +
+		"auto eth0\niface eth0 inet dhcp\n\n" +
+		"auto eth1\niface eth1 inet static\n    address 169.254.253.2/30\n"
+	if got := parseMode(interfaces, "eth0"); got != ModeDHCP {
+		t.Errorf("eth0 must classify DHCP even with an island eth1 static present, got %q", got)
+	}
+	if got := parseMode(interfaces, "eth1"); got != ModeStatic {
+		t.Errorf("eth1 (island) must classify static when asked directly (dev-scoped), got %q", got)
+	}
+}