server: refuse unsigned releases and polluted reserved addresses

Self-update now verifies SHA256SUMS.sig (ed25519, relsign package) against
a public key baked into the binary; the private key exists only in the CI
secret store, so a compromised release host can withhold updates but not
inject one. CI signs on every server-v* tag and hard-fails without the
secret. Operators with their own pipeline override the key via
ECHOLOT_SELF_UPDATE_PUBKEY (mint a pair with release-sign -gen).

Startup also now proves 80/443 are actually free on the reserved
measurement addresses by asking the OS (throwaway bind), not the config -
CheckReserved could never see a stray process, and the adb-beacon receiver
on 0.0.0.0:443 was exactly that.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/internal/selftest/reserved.go

@@ -0,0 +1,45 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package selftest
+
+import (
+	"fmt"
+	"net"
+)
+
+// ReservedWebPortsFree verifies that nothing on this host — this process or any other — is
+// listening on 80/443 of the reserved measurement addresses.
+//
+// config.CheckReserved keeps *our own* listeners off those ports, but our configuration is not
+// the host: the adb-beacon receiver, a separate Python process wildcard-bound to 0.0.0.0:443,
+// silently voided the IPv4 interception proof for as long as it ran, and nothing in this server's
+// config could have seen it. Asking the OS is the only check that covers processes we did not
+// start.
+//
+// The mechanism is a throwaway bind: if the bind succeeds the port was provably free (closed
+// again immediately — nothing is served), and if it fails with EADDRINUSE something is listening
+// there. Any other failure (address not assigned to this host, missing privilege) means the
+// question could not be answered, which is reported separately rather than pretending either way.
+func ReservedWebPortsFree(ips []net.IP) (occupied, unverifiable []string) {
+	return portsFree(ips, []string{"80", "443"})
+}
+
+func portsFree(ips []net.IP, ports []string) (occupied, unverifiable []string) {
+	for _, ip := range ips {
+		for _, port := range ports {
+			addr := net.JoinHostPort(ip.String(), port)
+			ln, err := net.Listen("tcp", addr)
+			if err == nil {
+				ln.Close()
+				continue
+			}
+			if isAddrInUse(err) {
+				occupied = append(occupied, addr)
+			} else {
+				unverifiable = append(unverifiable, fmt.Sprintf("%s (%v)", addr, err))
+			}
+		}
+	}
+	return occupied, unverifiable
+}