docs(server): record IPv6 zone identity as project policy

Endpoint identity documented the IPv6 scope identifier as an established
component alongside family, address and port. The server specification does
list the zone, but its remaining-unknowns section records that multi-zone
behavior could not be tested on a single-host platform, so no evidence settles
how a reference server compares scoped addresses. Presenting the choice as
verified compatibility overstated what is known, which the crate's own guidance
forbids.

Say what it is instead: a deliberate policy taken under a recorded unknown. Two
peers reachable at the same link-local address in different zones are genuinely
different peers, so treating the zone as identity is the conservative reading —
the failure it prevents is one peer closing another's session, and the failure
it risks is a scoped client having to reopen. The excluded flow label is
labelled the same way, and `irtt-server` guidance now carries the rule so the
echo slice reuses this comparison rather than inventing a second one.

Behavior is unchanged; this is documentation and a test comment.

crates/irtt-server/src/tests/close.rs

@@ -91,8 +91,11 @@ fn an_echo_after_a_close_is_a_stale_token() {
 
 #[test]
 fn a_close_from_any_other_endpoint_leaves_the_session_live() {
-    // Endpoint identity is exact: family, address, UDP source port and IPv6
-    // scope all count. Possessing the token is not authority to close.
+    // Possessing the token is not authority to close. The first four pairs
+    // differ in an established identity component; the last differs only in
+    // IPv6 zone, which is `irtt-rs` policy rather than observed behavior — see
+    // `same_endpoint`. It is pinned so the choice cannot drift silently, not
+    // because evidence settled it.
     for (bound, foreign) in [
         ("198.51.100.7:41234", "198.51.100.7:41235"),
         ("198.51.100.7:41234", "198.51.100.9:41234"),