Merge branch 'refactor-node' into refactor-node-next

Brings the handshake-leg deletion onto the XX line: the leg struct
and src/peer/connection.rs are gone, the Noise handles and crypto
methods now live on PeerMachine, and every leg accessor reads the
machine's own ConnectionState.

The incoming branch speaks Noise IK, this one speaks XX, so the crypto
methods could not be taken as they arrived. Git added the IK versions
to machine.rs with no conflict marker, and complete_handshake_msg3
-- which XX needs and IK has no counterpart for -- was absent
from the merged tree entirely. All four are re-expressed here: the
incoming structure (the Option<HandshakeCrypto> handle, the self.conn
reads, the borrow scoping that hoists carrier writes out of the leg
borrow) carrying this line's XX bodies. start_handshake is the one
that mattered most: its signature is identical on both lines, so it
compiled silently, and left alone it would have emitted an IK msg1
through the two-argument new_initiator and panicked every anonymous
dial on an expect for an identity XX does not have at that point. It
now uses the one-argument XX form and no expect.

handle_msg1 keeps this line's flow, not the incoming one's. The two
functions share a name but not a shape -- the admission gate moved
out of msg1 into msg3 when the line went to XX, and identity is
unknown until msg3. The machine is built above the crypto because
it now drives it, but it stays a local until the late insert, so a
rejected msg1 still leaves no registry trace and the drop-the-local
error arms are unchanged. Parking it at SentMsg2 after the index is
allocated needed a transition the birth constructor could not provide,
so that constructor now delegates to it.

Node::start_handshake is re-expressed the same way and for the same
reason: keeping the machine a local until all fallible setup has
succeeded preserves the existing error arms exactly, with no machine
disposal to add.

The two leg-to-carrier mirror blocks collapse. With one carrier
the writes they mirrored are self-assignment. Their content
is preserved: the completion touch comes from the relocated
complete_handshake, on the same clock, and the negotiated profile
from process_fmp_negotiation, which now takes the machine directly
and writes through set_conn_peer_profile.  Worth recording, because
the collapse was justified on a narrower claim that does not hold:
complete_handshake writes only the touch. It never wrote the profile
on either line. The two writes land on the same carrier at the same
point, so the collapse is neutral, but it is discharged jointly by
two methods rather than by complete_handshake alone.

Three inbound tests are dropped rather than carried. Each asserts
a property at msg1 that XX does not have there: an ACL decision, a
no-registry-trace guard on that decision, and an identity learn on the
carrier. Under XX msg1 is ephemeral-only, so none of the three has a
landing site at that step. A comment at each drop site records where
the property actually lives on this line. One of them was a deletion
here that the incoming branch had merely modified, so its removal
is this line's own prior decision rather than a new one. The same is
true of the craft_and_send_msg1 helper and of HandshakeSeed::inbound,
both left without callers once those tests went.

Also carried: peer_actions.rs had no conflict and no incoming change,
but three sites reached deleted Node accessors and are repointed at
the machine; the EstablishView snapshot is not resurrected, as this
line has never had it; and the connection-state doc keeps this line's
XX wording with the registry-independence invariant grafted in.

src/node/tests/session.rs

@@ -1005,9 +1005,7 @@ fn test_identity_cache_populated_on_promote() {
     let transport_id = TransportId::new(1);
     let link_id = LinkId::new(1);
 
-    let (conn, peer_identity) = make_completed_connection(&mut node, link_id, transport_id, 1000);
-
-    node.add_connection(conn).unwrap();
+    let peer_identity = seed_completed_connection(&mut node, link_id, transport_id, 1000);
 
     // Promote
     let result = node