Tailscale Root v0.6.4

Fixes since v0.6.3

• Route guard: determine the uplink from the main routing table instead of ip route get, which kept reporting a stale device after cellular handover.
• WebUI: pages now share one status snapshot, so tabs no longer diverge or re-poll on their own.

Assets: tailscale_root-v0.6.4.zip (module + arm64 binaries) and tailscale_root-v0.6.4-nobin.zip (module only).