Send fetch progress at most once per second (closes #206)

progressWriter sent an update for every chunk io.Copy wrote, so fetch
logged a line per 32 KiB. It now sends at most one per second, waiting
a second before the first as gen and check do, and one more when the
last byte is written.

TestFetchProgress now checks that a download of several chunks sends no
more than one update per second plus the last.

Model: opus-5-5

internal/cli/fetch_test.go

@@ -516,7 +516,7 @@ func TestManifestDownloadStopsAtLimit(t *testing.T) {
 func TestFetchProgress(t *testing.T) {
 	// Create source filesystem with a larger test file
 	sourceFs := afero.NewMemMapFs()
-	// Create content large enough to trigger multiple progress updates
+	// Content large enough that io.Copy writes it in at least four chunks
 	content := bytes.Repeat([]byte("x"), 100*1024) // 100KB
 	require.NoError(t, afero.WriteFile(sourceFs, "/large.txt", content, 0o644))
 
@@ -559,8 +559,11 @@ func TestFetchProgress(t *testing.T) {
 	}()
 
 	// Download
+	start := time.Now()
 	err = downloadFile(context.Background(), testClient(),
 		server.URL+"/large.txt", ".", "large.txt", files[0], progress)
+	elapsed := time.Since(start)
+
 	close(progress)
 	<-done
 
@@ -569,6 +572,12 @@ func TestFetchProgress(t *testing.T) {
 	// Verify we got progress updates
 	assert.NotEmpty(t, progressUpdates, "should have received progress updates")
 
+	// At most one update per second, plus the one reporting the file
+	// complete: a download that takes under a second sends two at most,
+	// however many chunks it writes.
+	assert.LessOrEqual(t, len(progressUpdates), 2+int(elapsed/time.Second),
+		"one progress update per second at most, plus the last")
+
 	// Verify final progress shows complete
 	if len(progressUpdates) > 0 {
 		last := progressUpdates[len(progressUpdates)-1]