node: skip parent explicitly in compute_mesh_size children loop

The mesh-size estimator's children loop relied on the cached
peer_declaration(parent_id).parent_id() != my_addr check to exclude
the parent. That cached view briefly disagrees with our own latest
my_declaration().parent_id() during the window between a local
parent-switch and the new parent's next inbound TreeAnnounce: the
peer-declaration cache still names us as the parent's parent, so the
parent is iterated as if it were a child and its (typically dominant)
bloom cardinality is added a second time. Symptom: estimated mesh size
displayed in fipsctl show status and fipstop nearly-but-not-exactly
doubles during tree rebalancing.

Make the invariant structural with an explicit peer_addr == parent_id
skip at the head of the children loop. Per-peer 500 ms rate-limiter
and overall recompute cadence are unchanged.

Adds a regression test that constructs the stale-peer-declaration
scenario directly and asserts the parent is not double-counted.

CHANGELOG.md

@@ -60,6 +60,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Mesh-size estimator (`compute_mesh_size`) no longer double-counts the
+  parent's bloom cardinality during the transient cache window after a
+  local parent-switch. Symptom: `fipsctl show status` / fipstop displayed
+  mesh size nearly-but-not-exactly doubling during tree rebalancing.
+  Fix: explicit parent-skip at the head of the children loop, making the
+  disjoint-subtree invariant structural rather than dependent on
+  `peer_declaration` cache freshness. Per-peer 500 ms rate-limiter and
+  overall recompute cadence are unchanged.
 - Spanning-tree state distribution is now eventually-consistent.
   Previously every `send_tree_announce_to_all` call site fired only
   on a local state-change event (parent switch, self-root promotion,