[5.6] KDBX compliance tests

[romejoe]

Summary

Automated test suite verifying .kdbx round-trip compatibility with major KeePass clients.

Requirements

• Test matrix:

  Operation	KeePassXC	KeePassDX	Strongbox	KeePassium
  Plugin creates → client opens
  Client creates → plugin opens
  Plugin edits → client verifies
  Client edits → plugin verifies
  Binary attachments round-trip
  Custom fields round-trip
  KDBX 4.x format
  KDBX 3.1 format

• Automated tests using kdbxweb to create/verify databases
• Manual test checklist for client-side verification
• CI integration: run automated tests on PRs

Acceptance Criteria

• All cells in the test matrix pass
• No data loss or corruption on round-trip
• Test suite runs in CI

Dependencies

• 1.2 (kdbxweb integration)
• 5.3 (attachment CRUD for binary round-trip tests)