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/relsign/relsign.go

@@ -0,0 +1,66 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Package relsign signs and verifies release manifests (detached ed25519 over SHA256SUMS).
+//
+// The checksum file alone protects download integrity, not authenticity: SHA256SUMS and the
+// binaries come from the same Gitea release, so whoever can alter one can alter both. The
+// signature is what separates "the file arrived intact" from "the project published this file" —
+// its private key lives in the CI secret store, not on the release host, so a compromised Gitea
+// can serve corrupted binaries but cannot make a self-updating server accept them.
+//
+// Formats, chosen to be reproducible with nothing but a stock library in any language:
+// the private key is the base64 of the 32-byte ed25519 seed, the public key the base64 of the
+// 32-byte public key, and the signature file the base64 of the 64-byte signature over the exact
+// bytes of the signed file.
+package relsign
+
+import (
+	"crypto/ed25519"
+	"encoding/base64"
+	"fmt"
+	"strings"
+)
+
+// GenerateKey mints a fresh signing keypair.
+func GenerateKey() (pubB64, seedB64 string, err error) {
+	pub, priv, err := ed25519.GenerateKey(nil)
+	if err != nil {
+		return "", "", err
+	}
+	return base64.StdEncoding.EncodeToString(pub),
+		base64.StdEncoding.EncodeToString(priv.Seed()), nil
+}
+
+// Sign produces the detached signature (base64) for data.
+func Sign(seedB64 string, data []byte) (string, error) {
+	seed, err := base64.StdEncoding.DecodeString(strings.TrimSpace(seedB64))
+	if err != nil {
+		return "", fmt.Errorf("signing key is not valid base64: %w", err)
+	}
+	if len(seed) != ed25519.SeedSize {
+		return "", fmt.Errorf("signing key must be %d bytes, got %d", ed25519.SeedSize, len(seed))
+	}
+	priv := ed25519.NewKeyFromSeed(seed)
+	return base64.StdEncoding.EncodeToString(ed25519.Sign(priv, data)), nil
+}
+
+// Verify checks a detached signature. A nil error means the holder of the private key matching
+// pubB64 signed exactly these bytes.
+func Verify(pubB64 string, data []byte, sigB64 string) error {
+	pub, err := base64.StdEncoding.DecodeString(strings.TrimSpace(pubB64))
+	if err != nil {
+		return fmt.Errorf("public key is not valid base64: %w", err)
+	}
+	if len(pub) != ed25519.PublicKeySize {
+		return fmt.Errorf("public key must be %d bytes, got %d", ed25519.PublicKeySize, len(pub))
+	}
+	sig, err := base64.StdEncoding.DecodeString(strings.TrimSpace(sigB64))
+	if err != nil {
+		return fmt.Errorf("signature is not valid base64: %w", err)
+	}
+	if !ed25519.Verify(ed25519.PublicKey(pub), data, sig) {
+		return fmt.Errorf("signature does not verify: the file was not signed by this key, or was altered after signing")
+	}
+	return nil
+}