Refuse a file whose size changed between freshen's scan and hash (closes #207)

freshen took a changed or new file's size from the scan and its hash
from a later read, without comparing the bytes read with that size. A
file that grew or shrank in between got an entry no content can match,
so check failed it with nothing saying why. processEntry now fails with
a size mismatch naming the file, as gen does, so freshen exits 1 and
leaves the manifest as it was.

Judgement call: the error wraps the package's existing errSizeMismatch
from fetch; its comment now covers both uses.

Model: opus-5-5

internal/cli/errmsg_test.go

@@ -473,14 +473,17 @@ func TestFreshenReadErrorMessage(t *testing.T) {
 		"read "+filepath.Join(root, "sub")+": is a directory")
 }
 
-// TestFreshenSizeChangedMessage has freshen hash a file that has grown
+// TestFreshenSizeChangedMessage has freshen hash a file that has grown or
-// since the scan recorded its size.
+// shrunk since the scan recorded its size.
 func TestFreshenSizeChangedMessage(t *testing.T) {
 	t.Parallel()
 
 	fs := afero.NewMemMapFs()
 	writeTestFile(t, fs, "/d/log.txt", "hello")
 
+	t.Run("grown", func(t *testing.T) {
+		t.Parallel()
+
 		hasher := &freshenHasher{
 			fs:      fs,
 			absBase: "/d",
@@ -493,6 +496,24 @@ func TestFreshenSizeChangedMessage(t *testing.T) {
 		assert.EqualError(t, err, `size mismatch for "log.txt": `+
 			`3 bytes when scanned but 5 bytes when read; `+
 			`the file changed while freshen ran`)
+	})
+
+	t.Run("shrunk", func(t *testing.T) {
+		t.Parallel()
+
+		hasher := &freshenHasher{
+			fs:      fs,
+			absBase: "/d",
+			builder: mfer.NewBuilder(),
+		}
+
+		err := hasher.processEntry(
+			&freshenEntry{path: "log.txt", size: 8, needsHash: true})
+		require.ErrorIs(t, err, errSizeMismatch)
+		assert.EqualError(t, err, `size mismatch for "log.txt": `+
+			`8 bytes when scanned but 5 bytes when read; `+
+			`the file changed while freshen ran`)
+	})
 }
 
 func TestURLRequiredMessage(t *testing.T) {