Skip files of an unreadable blob under restore --skip-errors (closes #218)

A blob that failed to download ended `snapshot restore` even with
--skip-errors, after restoring whichever files came first. The download
error now goes through the same per-file handling as any other restore
error, once for every pending file that references the blob. With
--skip-errors those files are reported as failed, the rest are
restored, and the command still exits non-zero. Without the flag the
restore still aborts; the error now also names one affected file and
suggests --skip-errors. A cancelled restore still ends at once. The
--skip-errors help text and README now limit the packing and storage
caveat to snapshot creation.

Model: opus-5-5

internal/vaultik/restore_interrupt_test.go

@@ -1,7 +1,6 @@
 package vaultik //nolint:testpackage // sets ctx/cancel and inspects scratch files
 
 import (
-	"bytes"
 	"context"
 	"io"
 	"path/filepath"
@@ -12,7 +11,6 @@ import (
 	"time"
 
 	"github.com/spf13/afero"
-	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/vaultik/internal/log"
 	"sneak.berlin/go/vaultik/internal/storage"
@@ -159,51 +157,3 @@ func scratchEntries(t *testing.T, dir string) []string {
 
 	return matches
 }
-
-// TestRestoreSkipErrorsCancelDuringBlobDownload cancels a SkipErrors
-// restore while a blob download is in progress. The download fails only
-// because of the cancel, so Restore must return context.Canceled without
-// reporting the file that needs the blob as failed.
-//
-//nolint:paralleltest // installs the global logger via log.Initialize
-func TestRestoreSkipErrorsCancelDuringBlobDownload(t *testing.T) {
-	log.Initialize(log.Config{})
-
-	fs := afero.NewOsFs()
-	tempDir := t.TempDir()
-
-	cfg, storer, snapshotID, srcPath := backupOneFile(context.Background(),
-		t, fs, tempDir, "a.txt", []byte("hello vaultik"), 0o644)
-
-	gate := newBlockingBlobStorer(storer)
-
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-
-	var out bytes.Buffer
-
-	v := newRestoreVaultik(ctx, cfg, gate, fs)
-	v.UI = ui.NewWithColor(&out, false)
-
-	restoreErr := make(chan error, 1)
-
-	go func() {
-		restoreErr <- v.Restore(&RestoreOptions{
-			SnapshotID: snapshotID,
-			TargetDir:  filepath.Join(tempDir, "restored"),
-			SkipErrors: true,
-		})
-	}()
-
-	select {
-	case <-gate.entered:
-	case <-time.After(30 * time.Second):
-		t.Fatal("restore never reached the blob-download phase")
-	}
-
-	cancel()
-
-	require.ErrorIs(t, <-restoreErr, context.Canceled)
-	assert.NotContains(t, out.String(), srcPath,
-		"the cancel was reported as a failed file")
-}