weston's VNC backend (neatvnc) uses VeNCrypt (security type 19) as
the outer type even with --disable-transport-layer-security, offering
sub-type 1 (None, no TLS, no password) within it.
The old screen.html only handled type 1 (plain None) and type 2 (VNC
auth), causing 'auth failed' against weston.
Add VeNCrypt states to the RFB state machine:
- vencrypt-version: echo back server's major.minor
- vencrypt-subtypes: pick sub-type 1 (None) if available
- vencrypt-accept: check server's acceptance byte
Then falls through to the normal security-result / server-init path.
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