Refuse a file entry with no hash when loading a manifest (closes #194)

Loading accepted a file entry whose hashes list was empty. check and
fetch then failed the file as a hash mismatch, and freshen left it out
of the rewritten manifest without a word.

NewManifestFromReader now refuses such an entry, naming its path, after
checking the path itself. freshen's addExistingToBuilder returns an
error for an entry with no hash instead of skipping it. The file entry
table in docs/FORMAT.md says readers reject an entry with none.

Model: opus-5-5

internal/cli/freshen_test.go

@@ -471,6 +471,24 @@ func TestFreshenAddExistingRejectsMissingMtime(t *testing.T) {
 	assert.Contains(t, err.Error(), "file1.txt")
 }
 
+// TestFreshenAddExistingRejectsMissingHash pins that an entry with no hash
+// is an error rather than a file left out of the rebuilt manifest.
+func TestFreshenAddExistingRejectsMissingHash(t *testing.T) {
+	t.Parallel()
+
+	b := mfer.NewBuilder()
+	entry := &mfer.MFFilePath{
+		Path:  "data.bin",
+		Size:  8,
+		Mtime: &mfer.Timestamp{Seconds: 1_700_000_000},
+	}
+
+	err := addExistingToBuilder(b, entry)
+	require.ErrorIs(t, err, errEntryMissingHash)
+	assert.Contains(t, err.Error(), "data.bin")
+	assert.Zero(t, b.FileCount())
+}
+
 // TestFreshenAddExistingRejectsShortHash pins that an existing manifest
 // entry whose hash the builder refuses is reported as a problem with the
 // manifest, with the command that fixes it.