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.

src/node/mod.rs

@@ -1173,6 +1173,9 @@ impl Node {
 
         // Children's filters: each child's subtree is disjoint
         for (peer_addr, peer) in &self.peers {
+            if peer_addr == &parent_id {
+                continue;
+            }
             if let Some(decl) = self.tree_state.peer_declaration(peer_addr)
                 && *decl.parent_id() == my_addr
             {