docs(runbook): assert waiter identity, not liveness, in the reachability audit

The cross-reference loop filtered live waiters on `kill -0 "$pid"`. A lock
file left by a reaped listener names a pid the kernel is free to reissue,
so the loop would report an unrelated process as a live waiter — and a
phantom waiter is how a false "your seat is unreachable" notice reaches a
seat that is fine, with nothing in the output to falsify it.

`session_listener.sh --stop` already refuses this standard: it walks
/proc/<pid>/cmdline for the `--_route=<handle>` segment before signalling,
on the grounds that a live pid proves existence and not identity. The audit
now asserts the same thing and prints STRANGER on a mismatch.

Verified against the live box: six waiters, all identity-confirmed, all
reporting push.

docs/runbooks/althing-deploy.md

@@ -99,16 +99,33 @@ what the post office believes:
 for f in ~/.althing/wake-listener-*.lock; do
   h=$(basename "$f" .lock); h=${h#wake-listener-}
   pid=$(cat "$f" 2>/dev/null)
-  if [ -n "$pid" ] && kill -0 "$pid" 2>/dev/null; then
-    printf "  %-24s waiter %-8s mode: %s\n" "$h" "$pid" \
-      "$(postbox status --handle "$h" 2>/dev/null | grep -oP 'mode: \K\w+')"
-  fi
+  [ -n "$pid" ] && [ -r "/proc/$pid/cmdline" ] || continue
+  # IDENTITY, not liveness: a lock left by a reaped listener names a pid the
+  # kernel is free to hand to anything, so `kill -0` alone reports a stranger
+  # as a live waiter. Match the same cmdline segment `althing-listen --stop`
+  # requires before it will signal anything.
+  ours=0
+  while IFS= read -r -d '' seg; do [ "$seg" = "--_route=$h" ] && ours=1; done \
+    < "/proc/$pid/cmdline"
+  [ "$ours" = 1 ] || { printf "  %-24s pid %-8s STRANGER (recycled pid)\n" "$h" "$pid"; continue; }
+  printf "  %-24s waiter %-8s mode: %s\n" "$h" "$pid" \
+    "$(postbox status --handle "$h" 2>/dev/null | grep -oP 'mode: \K\w+')"
 done
 ```
 
 A live waiter reporting `mode: pull` is a seat that will never be poked. From
 3.2.4 onward `$ALTHING_ROOT/listen.log` records failed declarations directly.
 
+⚠ **Liveness is not identity, and this loop is the place that gets it wrong.**
+The earlier `kill -0` form would print a phantom waiter for any handle whose
+stale lock happens to name a recycled pid — and a phantom waiter is exactly what
+sends a false "you are unreachable" notice to a seat that is fine.
+`session_listener.sh` refuses to SIGTERM on liveness alone for precisely this
+reason; an audit that only *reads* has no excuse for a weaker standard than the
+one that *kills*. (Adopted 2026-09-02 after the regin-smithy-dev
+cross-reference, where the pid happened to be genuine and the weaker check
+happened to be right.)
+
 ⚠ **A stale `wake-listener-*.lock` is NOT a fault.** The gate is `flock -n` on an
 open fd, which the kernel releases when the holder dies, so a lock file left by a
 reaped listener is inert and exit 3 only fires against a genuinely live holder.