fix: report reconciliation publication state accurately

[codeforester]

Fixes #46. Separates atomic publication from durability confirmation and reports post-replacement failures without claiming old bytes survived. Tests: tests/test_cli.py.

[codeforester]

Review: logic is sound: _PublishedSnapshotDurabilityError subclasses OSError and is caught before the generic handler, staged_path is cleared once os.replace succeeds, and the file-fsync-failure path still reports 'previous snapshot left unchanged'. Ready to merge. Optional follow-ups: no test covers the Windows return False branch (warning logged, exit 0); and test_directory_fsync_failure_reports_published_snapshot is missing a blank line before the nested def.