Find every copy: an erasure dry run

The tool has two halves that must not share assumptions. erase_matter uses the design's keys (matter id, run id, content hash) to delete what the matter owns. verify_erased ignores all of that and scans every cell, line, blob, tar member and live Redis key for the matter's identifiers. The gap between them is the finding: whatever the scan sees after a delete-by-ID pass is a copy the design made without meaning to. The project is standard-library Python only, so it runs anywhere uv does.