Compare --require-signature with the key that signed (closes #167)

check and fetch --require-signature compared the required fingerprint
with the first key in the manifest's embedded public key block, while
gpg accepted a good signature by any key in that block.

Loading a signed manifest now refuses one whose embedded block holds
more than one primary key, or whose signer field is not the fingerprint
gpg reports for the signing key on its VALIDSIG status line.
--require-signature compares with the signer field, which loading has
checked. Signing now signs with and exports the key by its fingerprint,
so a key ID matching two keys still writes a manifest that loads.
docs/FORMAT.md states what a verifier checks.

Model: opus-5-5

internal/cli/check.go

@@ -126,10 +126,8 @@ func (mfa *CLIApp) fetchManifestToTemp(
 }
 
 // verifyRequiredSigner enforces the --require-signature fingerprint
-// against the manifest's embedded signing key.
-func verifyRequiredSigner(
-	ctx context.Context, chk *mfer.Checker, requiredSigner string,
-) error {
+// against the key that made the manifest's signature.
+func verifyRequiredSigner(chk *mfer.Checker, requiredSigner string) error {
 	// Validate fingerprint format: must be exactly 40 hex characters
 	if len(requiredSigner) != fingerprintHexLen {
 		return fmt.Errorf("%w, got %d", errInvalidFingerprint, len(requiredSigner))
@@ -145,22 +143,17 @@ func verifyRequiredSigner(
 			errManifestNotSigned, requiredSigner)
 	}
 
-	// Extract fingerprint from the embedded public key (not from the
-	// signer field). This validates the key is importable and gets its
-	// actual fingerprint.
-	embeddedFP, err := chk.ExtractEmbeddedSigningKeyFP(ctx)
-	if err != nil {
-		return fmt.Errorf(
-			"failed to extract fingerprint from embedded signing key: %w", err)
-	}
+	// Loading the manifest checked that the signer is the fingerprint of
+	// the key that made the signature.
+	signer := string(chk.Signer())
 
 	// Compare fingerprints - must be exact match (case-insensitive)
-	if !strings.EqualFold(embeddedFP, requiredSigner) {
+	if !strings.EqualFold(signer, requiredSigner) {
 		return fmt.Errorf("embedded signing key fingerprint %s %w %s",
-			embeddedFP, errSignerMismatch, requiredSigner)
+			signer, errSignerMismatch, requiredSigner)
 	}
 
-	log.Infof("manifest signature verified (signer: %s)", embeddedFP)
+	log.Infof("manifest signature verified (signer: %s)", signer)
 
 	return nil
 }
@@ -330,7 +323,7 @@ func (mfa *CLIApp) checkManifestOperation(
 	// Check signature requirement
 	requiredSigner := cmd.String(flagRequireSignature)
 	if requiredSigner != "" {
-		err = verifyRequiredSigner(ctx, chk, requiredSigner)
+		err = verifyRequiredSigner(chk, requiredSigner)
 		if err != nil {
 			return err
 		}