[Phase 2 / Feature B / B-9] Interop test: KeePassXC round-trip with NKW.Passkey.* fields

[gynet]

Parent: #9. Depends on #52 (B-8). Owner: SDET.

Scope

Extend the keepass-rs interop harness (in `packages/db/test/`) with a test that:

1. NeoKeeWeb creates a passkey entry (Feature B create flow, mocked via direct bridge call)
2. KDBX saved
3. keepass-rs (Rust harness) opens the file, reads the entry, asserts all 13 NKW.Passkey.* fields present
4. keepass-rs modifies some unrelated field (e.g., entry notes), saves
5. NeoKeeWeb re-opens, `passkey-get-by-rp` still works, SignCount preserved
6. Actually sign with the re-opened key, verify against stored public key

Why it matters

Interop breakage here = we silently destroyed user data for KeePassXC users. This test is the non-negotiable gate for Feature B.

Acceptance

• Test added to interop harness
• Protected fields survive KeePassXC save cycle
• SignCount survives
• Public key + signature verify post-roundtrip