fix(backup): let the stopped copy replace a failed live pre-copy

A volume whose containers must stop was copied twice: live first, then
with the containers stopped and --checksum. A live writer that truncates
a file under rsync (read errors mapping ...: No data available (61),
exit 23) aborted the run in the first pass, before the containers were
stopped, although the second pass rewrites every differing file anyway.
Seen in infinito-nexus CI on kix_kix_shared/objecticons.

The live copy now only warns when a stopped copy follows. Without a stop
it stays the only copy and still fails the run, and a failing stopped
copy still fails it.

The e2e test drives a writer that shrinks files mid-read; without the
fix it reproduces the CI failure verbatim.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/baudolo/backup/app.py

@@ -23,6 +23,7 @@ from .layout import (
     write_manifest,
 )
 from .policy import requires_stop, volume_is_fully_ignored
+from .shell import BackupError
 from .snapshot import snapshot_source, volume_snapshot
 from .volume import backup_volume, inspect_backing
 
@@ -125,13 +126,23 @@ def main() -> int:
                     copy(authoritative=False)
                 continue
 
-            copy(authoritative=False)
-            if requires_stop(containers, args.images_no_stop_required):
-                stoppable = filter_stoppable(containers)
-                change_containers_status(stoppable, "stop")
-                copy(authoritative=True)
-                if not args.shutdown:
-                    change_containers_status(stoppable, "start")
+            if not requires_stop(containers, args.images_no_stop_required):
+                copy(authoritative=False)
+                continue
+
+            try:
+                copy(authoritative=False)
+            except BackupError as error:
+                print(
+                    f"WARNING: live pre-copy of volume '{volume_name}' failed; "
+                    f"the copy with its containers stopped replaces it.\n{error}",
+                    flush=True,
+                )
+            stoppable = filter_stoppable(containers)
+            change_containers_status(stoppable, "stop")
+            copy(authoritative=True)
+            if not args.shutdown:
+                change_containers_status(stoppable, "start")
 
     write_manifest(version_dir, outcomes)
     stamp_directory(version_dir)