fix(snapshot): refuse receipt paths that escape the handed directory

[VARUN3WARE]

Summary

• Closes [Bug]: Snapshot verify can read outside the handed directory via receipt paths #469: verify_dataset_snapshot reuses _validated_relative_key and raises (CLI exit 2) before reading absolute / .. / drive-letter receipt paths.
• Threat model: not local privilege escalation; restores the documented “joined only onto the handed directory” promise for markers we did not write (tenant-supplied snapshots).
• Built on the fix(snapshot): refuse markers whose format identity the exporter would refuse #486 / fix(snapshot): enforce the receipt inventory content_id at verify time #483 boundary gates already on main.

Test plan

• Relative ../, absolute path, and tables/../../… (subdir present) → exit 2
• Honest relative export still verifies clean
• tests/test_snapshot_verify.py + tests/test_dataset_snapshot.py
• CI green