Require a SHA-256 hash in every manifest entry (closes #224)

Loading accepted an entry whose hashes held no SHA-256 multihash, and
check and fetch, which compute only SHA-256, then reported the intact
file as a hash mismatch. Loading now refuses, naming the path, an entry
that does not list exactly one SHA-256 multihash; other hashes beside
it are allowed. AddFileWithHash accepts only a SHA-256 multihash, and
freshen carries an unchanged entry's SHA-256 hash over instead of its
first hash, found with SHA256Multihash, new in package mfer.
docs/FORMAT.md states the rule for writers and readers. Two loading
tests whose entries held an empty hash now give each entry a SHA-256
hash; the decoding-size test's path lengths change with it.

Model: opus-5-5

docs/FORMAT.md

@@ -80,15 +80,15 @@ After decompressing `innerMessage`, the result is a serialized `MFFile`
 
 Each file entry contains:
 
-| Field      | Number | Type                      | Description                                                   |
-| ---------- | ------ | ------------------------- | ------------------------------------------------------------- |
-| `path`     | 1      | string                    | Relative file path (see Path Rules)                           |
-| `size`     | 2      | int64                     | File size in bytes                                            |
-| `hashes`   | 3      | repeated `MFFileChecksum` | At least one hash required; readers reject an entry with none |
-| `mimeType` | 301    | string (optional)         | MIME type                                                     |
-| `mtime`    | 302    | Timestamp (optional)      | Modification time                                             |
-| `ctime`    | 303    | Timestamp (optional)      | Change time (inode metadata change)                           |
-| `mode`     | 304    | uint32                    | Permission bits (see File Mode)                               |
+| Field      | Number | Type                      | Description                                                  |
+| ---------- | ------ | ------------------------- | ------------------------------------------------------------ |
+| `path`     | 1      | string                    | Relative file path (see Path Rules)                          |
+| `size`     | 2      | int64                     | File size in bytes                                           |
+| `hashes`   | 3      | repeated `MFFileChecksum` | Must include exactly one SHA-256 multihash (see Hash Format) |
+| `mimeType` | 301    | string (optional)         | MIME type                                                    |
+| `mtime`    | 302    | Timestamp (optional)      | Modification time                                            |
+| `ctime`    | 303    | Timestamp (optional)      | Change time (inode metadata change)                          |
+| `mode`     | 304    | uint32                    | Permission bits (see File Mode)                              |
 
 ## File Mode
 
@@ -133,8 +133,16 @@ Each checksum is a single `bytes multiHash` field containing a
 self-describing: the encoded bytes include a varint algorithm identifier
 followed by a varint digest length followed by the digest itself.
 
-The 1.0 implementation writes SHA-256 multihashes (`0x12` algorithm code).
-Implementations must be able to verify SHA-256 multihashes at minimum.
+A writer must give every file entry exactly one SHA-256 multihash of the file's
+content: the algorithm code `0x12`, the digest length `0x20` and the 32 bytes of
+the digest. It may list multihashes of other algorithms beside it. A reader must
+refuse a manifest with an entry that has no SHA-256 multihash or more than one,
+whatever other hashes the entry lists. A verifier then needs only SHA-256,
+checking each file against its entry's SHA-256 multihash; it may ignore the
+other hashes.
+
+The reference implementation writes one hash for each entry, the SHA-256
+multihash, and its `check` and `fetch` compute only SHA-256.
 
 ## Signature Scheme
 
@@ -212,10 +220,11 @@ compresses the same inner message to the same bytes:
 - Each entry holds only `path`, `size`, `hashes` with exactly one hash, `mtime`
   and `mode`. The hash is the SHA-256 multihash of the file's content, except
   that `freshen` reads only the files that are new or whose size, `mtime` or
-  `mode` changed, and for every other file carries the first hash of its
-  existing entry over unchanged. `mtime` is the file's modification time as the
-  file system gives it, to the nanosecond, and `mode` is `0000` unless `gen` or
-  `freshen` is given `--include-permissions`.
+  `mode` changed, and for every other file carries the SHA-256 multihash of its
+  existing entry over unchanged, leaving out any other hash the entry lists.
+  `mtime` is the file's modification time as the file system gives it, to the
+  nanosecond, and `mode` is `0000` unless `gen` or `freshen` is given
+  `--include-permissions`.
 - `createdAt` is left out unless `gen` or `freshen` is given
   `--include-timestamps`. Then it holds the time the run started building the
   manifest, to the nanosecond, so it differs on every run.