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

docs/FORMAT.md

@@ -37,7 +37,7 @@ The outer message contains:
 | `uuid`            | 105    | bytes            | Random v4 UUID; must match the inner message UUID                        |
 | `innerMessage`    | 199    | bytes            | Zstd-compressed serialized `MFFile` message                              |
 | `signature`       | 201    | bytes (optional) | GPG signature (ASCII-armored or binary)                                  |
-| `signer`          | 202    | bytes (optional) | Full GPG key ID of the signer                                            |
+| `signer`          | 202    | bytes (optional) | Fingerprint of the signing key                                           |
 | `signingPubKey`   | 203    | bytes (optional) | Full GPG signing public key                                              |
 
 ### SHA-256 Hash
@@ -137,7 +137,16 @@ Where:
   compressed data)
 
 Components are separated by hyphens. The signature is produced by GPG over this
-canonical string and stored in the `signature` field of the outer message.
+canonical string and stored in the `signature` field of the outer message. The
+signing key's public key goes in `signingPubKey` and its fingerprint, in hex, in
+`signer`.
+
+A verifier accepts a signed manifest only if `signingPubKey` holds exactly one
+primary key, `signature` is one good signature over the canonical string made by
+that key (or one of its subkeys), and `signer` is that key's fingerprint. The
+reference implementation refuses to load a manifest that fails these checks;
+`check` and `fetch` given `--require-signature` then compare the required
+fingerprint with `signer`.
 
 ## Deterministic Serialization