[Pithos Lib] Reject invalid directory markers and CRCs

[das-Abroxas]

Goal

Corrupted directory metadata should be detected reliably. Marker and CRC damage must not be accepted silently or fail in surprising ways.

Workplan

1. Identify existing directory parsing and validation behavior for markers and CRCs.
2. Add permanent malformed-directory regression fixtures or generated test cases.
3. Reject corrupted directory records wherever they are currently accepted.

Definition of Done

• Corrupted directory markers are covered by regression tests.
• Corrupted directory CRC values are covered by regression tests.
• Invalid directory records return clear errors.

Test Concept

• Add marshalling or reader tests that corrupt directory marker bytes and CRC fields in otherwise valid archives.
• Verify corrupted archives fail deterministically without panics.