[Bug]: Incomplete protected backups are reported as successful

[oleksii-tumanov]

What happened?

A failed graph.json backup copy can still print backed up semantic graph and return the backup directory. In a local reproduction, a read-only same-day backup kept the old graph while a sidecar copied successfully. There was no warning that the returned backup was stale.

I'd expect an incomplete backup to return None and warn which file failed. The existing best-effort overwrite behavior can stay unchanged.

Steps to reproduce

Run as a regular user on macOS or Linux; root may bypass the read-only permission:

python - <<'PY'
from contextlib import redirect_stderr, redirect_stdout
from io import StringIO
from pathlib import Path
from tempfile import TemporaryDirectory
from graphify.export import backup_if_protected

with TemporaryDirectory() as temporary:
    out = Path(temporary)
    graph = out / "graph.json"
    graph.write_text('{"nodes":[{"id":"old"}],"links":[]}')
    (out / ".graphify_semantic_marker").write_text("{}")
    with redirect_stdout(StringIO()):
        backup = backup_if_protected(out)
    protected = backup / "graph.json"
    protected.chmod(0o444)
    try:
        graph.write_text('{"nodes":[{"id":"new"}],"links":[]}')
        stdout, stderr = StringIO(), StringIO()
        with redirect_stdout(stdout), redirect_stderr(stderr):
            result = backup_if_protected(out)
        print("result_is_none:", result is None)
        print("backup_matches_current_graph:", protected.read_bytes() == graph.read_bytes())
        print("stdout:", stdout.getvalue().strip())
        print("stderr:", stderr.getvalue().strip().replace(temporary, "<temporary-output>"))
    finally:
        protected.chmod(0o644)
PY

Error output or graph output

Observed on the tested base:

result_is_none: False
backup_matches_current_graph: False
stdout: [graphify] backed up semantic graph (1 files) -> 2026-09-22/
stderr:

Graphify version

0.9.66, v8 at a5957aa6ef51c9be8d054de9783d25046c187f3f.

Environment

macOS 26.6.2, Python 3.12.11, editable install.

Additional context

This is about copy-failure reporting and retrying incomplete same-day backups, not changing backup eligibility or retention.