Removes weston-rdp.nix (hyperhive.westonRdp.enable) and adds
weston-vnc.nix (hyperhive.gui.enable).
The ExecStart wrapper script computes a deterministic VNC port via
FNV-1a hash of the agent name (derived from hostname, same algorithm
as lifecycle::agent_web_port) in the range [15900, 16799], then writes
/etc/hyperhive/gui.json {"vnc_port": N, "auth": "none"} for the
harness WebSocket relay (issue #51), and execs weston with the VNC
backend + pixman renderer.
Type=simple so it can never abort nixos-container update; a
misconfigured weston degrades to a restart loop, not a blocked rebuild.
Closes#50
