Every fc.last_snapshot_hash assignment across handlers_fix.py, handlers_build.py,
and handlers_edit.py now sets fc.last_structure_fingerprint alongside it, using
result.structure_fingerprint at apply-repair sites and
env.dify.structural_fingerprint(graph) at read-graph/restore sites.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>