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/AGENTS.md

@@ -25,6 +25,14 @@ material does not establish a semantic, state the ambiguity instead of guessing.
 - No statistics, CLI or presentation concerns.
 - `SocketAddr` belongs here: endpoint identity is session state. It does not
   belong in `irtt-proto`.
+- Endpoint identity is compared field by field, not with `SocketAddr` equality.
+  Family, address and port are established components. The IPv6 flow label is
+  excluded because it identifies no endpoint; the IPv6 zone is included as a
+  deliberate project policy, because the specification records multi-zone
+  behavior as untested and two peers sharing a link-local address in different
+  zones are genuinely different peers. Reuse that one comparison rather than
+  writing a second identity rule, and keep both choices labelled as policy
+  rather than as verified compatibility.
 
 ## Core versus runtime