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/session.rs

@@ -27,11 +27,17 @@ impl Session {
 
     /// The exact source endpoint the open request arrived from.
     ///
-    /// A session is bound to this endpoint, not merely to its address: address
-    /// family, address, UDP source port and — for IPv6 — the scope identifier
-    /// all form part of its identity. Close processing requires both a token
-    /// match and an exact endpoint match before removing a session, and echo
-    /// processing will require the same.
+    /// A session is bound to this endpoint, not merely to its address: the
+    /// address family, the address and the UDP source port all form part of its
+    /// identity, as — by `irtt-rs` policy rather than observed behavior — does
+    /// the IPv6 scope identifier. Close processing requires both a token match
+    /// and an endpoint match before removing a session, and echo processing
+    /// will require the same; `same_endpoint` in `core` is where the comparison
+    /// and its reasoning live.
+    ///
+    /// The address is stored exactly as received, including the flow label that
+    /// identity ignores, so a reply goes back to the endpoint the request
+    /// actually came from.
     pub(crate) fn peer(&self) -> SocketAddr {
         self.peer
     }