Expand manual security test matrix for public releases

[boggspa]

Broaden the release test matrix before the next public preview.

Acceptance criteria:

• Native Screen Q pairing, encryption, and permission gates are tested.
• Mac Screen Sharing and Generic VNC credential boundaries are tested.
• RDP certificate pinning and changed-certificate handling are tested.
• File transfer rejects traversal, ordering, and oversize cases.
• Results are linked from Docs/ReleaseHardening.md.