peer: move the Noise handshake operations onto the control machine

The pending connection drove its own Noise handshake while the control
machine held it, so the crypto and the state it produces lived on the
carrier that is going away. Move the six operations onto the machine:
starting and completing an initiation, processing an inbound
initiation, taking the session, testing for one, and dropping the
handle on failure. Bodies are unchanged apart from reaching the
handles through the attached connection.

Each operation now records its results on both carriers. The learned
identity, the remote epoch, and the activity stamp are written to the
machine's own bookkeeping at the same point and with the same value as
they are written to the connection's. The connection's copies still
have readers until those reads are repointed, so both have to be
written; the machine's copy of the learned identity was previously
never populated for an inbound connection, which is why an inbound
pending row showed no expected peer.

Driving the handshake from the machine means the machine has to exist
before the crypto runs. On the inbound path it is built above the
message-1 processing and still kept local, so a rejected message
leaves no registry entry and allocates no index. On the outbound path
it already existed from the dial, so it simply takes the connection
before the index allocation, and both failure arms unwind it as they
did.

Completing message 2 no longer mirrors the activity stamp separately,
since the completion itself now writes both carriers at the point the
mirror was approximating.

Three tests cover what the compiler cannot. A connection whose
handshake failed holds neither Noise handle yet must stay visible to
the stale-connection sweep, or every failed connection would leak and
hold a peering-budget slot forever. A message 1 rejected by the crypto
or by the ACL must leave no machine, no index, and the same rejection
count as before. A dial whose message-1 preparation fails must unwind
the machine registered at dial time; that test drives the
index-allocation failure rather than a crypto failure, which is the
arm this change actually widens, since the allocation now happens with
the connection already attached.

src/node/tests/spanning_tree.rs

@@ -131,7 +131,8 @@ pub(super) async fn initiate_handshake(nodes: &mut [TestNode], i: usize, j: usiz
     let startup_epoch = initiator.node.startup_epoch();
     let noise_msg1 = initiator
         .node
-        .get_connection_mut(&link_id)
+        .peer_machines
+        .get_mut(&link_id)
         .unwrap()
         .start_handshake(our_keypair, startup_epoch, 1000)
         .unwrap();