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/entry_test.go

@@ -613,6 +613,33 @@ func runCheckAfterRewrite(t *testing.T, rewritten, msg string) {
 	assert.Equal(t, 1, exitCode, msg)
 }
 
+// TestCheckRequireSignatureRefusesOtherSigningKey runs check
+// --require-signature on a manifest signed by another key whose embedded
+// public key block also holds the required key. check must refuse it. It
+// needs gpg and is skipped without it, as the other signing tests are.
+//
+//nolint:paralleltest // signedManifest calls t.Setenv, which bars t.Parallel
+func TestCheckRequireSignatureRefusesOtherSigningKey(t *testing.T) {
+	content := []byte("signed file")
+	manifest, required := manifestSignedByAnotherKey(t,
+		map[string][]byte{testFileTxt: content})
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, fs.MkdirAll(testDir, 0o755))
+	require.NoError(t, afero.WriteFile(fs,
+		filepath.Join(testDir, testFileTxt), content, 0o644))
+	require.NoError(t, afero.WriteFile(fs, testManifest, manifest, 0o644))
+
+	opts := testOpts([]string{
+		testApp, cmdCheck, "-q", testFlagBase, testDir,
+		"--" + flagRequireSignature, required, testManifest,
+	}, fs)
+	assert.Equal(t, 1, runCLI(opts))
+	assert.Contains(t, testStderr(t, opts),
+		"failed to load manifest: signature verification failed: "+
+			"embedded public key block must hold exactly one key, found 2")
+}
+
 func TestCheckCommandWithCorruptedFile(t *testing.T) {
 	t.Parallel()