Tailscale accepts both named ("tcp") and numeric IANA ("6") protocol
forms wherever a Protocol value is allowed. Headscale stored whichever
form the user wrote, leaving downstream code with two equivalents to
handle separately. validateProtocolPortCompatibility only recognised
the named constants and rejected the numeric form, so a policy with
`proto: "6", dst: ["host:443"]` was rejected at parse time even though
SaaS accepts it.
Resolve the disagreement by normalising to the named form during
Protocol.UnmarshalJSON. Every downstream consumer now sees one form
regardless of what the user wrote, so layered guards like
`|| protocol == "6"` in the validator are unnecessary.
Updates #1803
e4e209f919