feat(manifest): add ed25519 signing and verification

Sign() computes signature over canonical YAML (excluding sign field),
Verify() checks against public key. Tampered manifests are rejected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/manifest/sign.go

@@ -0,0 +1,43 @@
+package manifest
+
+import (
+	"crypto/ed25519"
+	"encoding/base64"
+	"fmt"
+
+	"gopkg.in/yaml.v3"
+)
+
+// signable returns the canonical bytes to sign (manifest without sign field).
+func signable(m *Manifest) ([]byte, error) {
+	tmp := *m
+	tmp.Sign = ""
+	return yaml.Marshal(&tmp)
+}
+
+// Sign computes the ed25519 signature and stores it in m.Sign (base64).
+func Sign(m *Manifest, priv ed25519.PrivateKey) error {
+	msg, err := signable(m)
+	if err != nil {
+		return fmt.Errorf("manifest.Sign: marshal: %w", err)
+	}
+	sig := ed25519.Sign(priv, msg)
+	m.Sign = base64.StdEncoding.EncodeToString(sig)
+	return nil
+}
+
+// Verify checks the ed25519 signature in m.Sign against the public key.
+func Verify(m *Manifest, pub ed25519.PublicKey) (bool, error) {
+	if m.Sign == "" {
+		return false, fmt.Errorf("manifest.Verify: no signature present")
+	}
+	sig, err := base64.StdEncoding.DecodeString(m.Sign)
+	if err != nil {
+		return false, fmt.Errorf("manifest.Verify: decode: %w", err)
+	}
+	msg, err := signable(m)
+	if err != nil {
+		return false, fmt.Errorf("manifest.Verify: marshal: %w", err)
+	}
+	return ed25519.Verify(pub, msg, sig), nil
+}

pkg/manifest/sign_test.go

@@ -0,0 +1,51 @@
+package manifest
+
+import (
+	"crypto/ed25519"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSignAndVerify_Good(t *testing.T) {
+	pub, priv, err := ed25519.GenerateKey(nil)
+	require.NoError(t, err)
+
+	m := &Manifest{
+		Code:    "test-app",
+		Name:    "Test App",
+		Version: "1.0.0",
+		Layout:  "HLCRF",
+		Slots:   map[string]string{"C": "main"},
+	}
+
+	err = Sign(m, priv)
+	require.NoError(t, err)
+	assert.NotEmpty(t, m.Sign)
+
+	ok, err := Verify(m, pub)
+	require.NoError(t, err)
+	assert.True(t, ok)
+}
+
+func TestVerify_Bad_Tampered(t *testing.T) {
+	pub, priv, _ := ed25519.GenerateKey(nil)
+	m := &Manifest{Code: "test-app", Version: "1.0.0"}
+	_ = Sign(m, priv)
+
+	m.Code = "evil-app" // tamper
+
+	ok, err := Verify(m, pub)
+	require.NoError(t, err)
+	assert.False(t, ok)
+}
+
+func TestVerify_Bad_Unsigned(t *testing.T) {
+	pub, _, _ := ed25519.GenerateKey(nil)
+	m := &Manifest{Code: "test-app"}
+
+	ok, err := Verify(m, pub)
+	assert.Error(t, err)
+	assert.False(t, ok)
+}