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

@@ -473,6 +473,28 @@ func TestFreshenReadErrorMessage(t *testing.T) {
 		"read "+filepath.Join(root, "sub")+": is a directory")
 }
 
+// TestFreshenSizeChangedMessage has freshen hash a file that has grown
+// since the scan recorded its size.
+func TestFreshenSizeChangedMessage(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	writeTestFile(t, fs, "/d/log.txt", "hello")
+
+	hasher := &freshenHasher{
+		fs:      fs,
+		absBase: "/d",
+		builder: mfer.NewBuilder(),
+	}
+
+	err := hasher.processEntry(
+		&freshenEntry{path: "log.txt", size: 3, needsHash: true})
+	require.ErrorIs(t, err, errSizeMismatch)
+	assert.EqualError(t, err, `size mismatch for "log.txt": `+
+		`3 bytes when scanned but 5 bytes when read; `+
+		`the file changed while freshen ran`)
+}
+
 func TestURLRequiredMessage(t *testing.T) {
 	t.Parallel()