Make the overlay dual stack

Every member now also derives an IPv4 address, from the same inputs as its
IPv6 one, into 100.64.0.0/10 by default. The range is configurable and IPv4
can be turned off with --no-ipv4.

IPv4 is honestly weaker than IPv6 here and the code says so. A 64 bit
interface identifier makes an IPv6 collision impossible in practice; IPv4
has nothing like that room, and in a /10 with 50 members two will derive the
same address about 0.03% of the time. A mesh with no coordinator cannot
allocate around that, so a collision is detected and resolved instead: the
member whose public key sorts lower keeps the address, a rule every member
computes identically and therefore agrees on. The other keeps IPv6 and is
flagged in the status. IPv6 always works; IPv4 almost always works and
degrades predictably.

Routing and address-ownership enforcement now cover both families: a packet
goes to the peer that owns its destination, and a decrypted packet is
dropped unless its source is an address derived for the peer that sent it,
IPv4 included.

Six new tests, among them a real IPv4 packet crossing a tunnel next to an
IPv6 one, a spoofed IPv4 source being dropped, and an IPv6-only overlay.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/testing.md

@@ -50,6 +50,11 @@ keeping the WireGuard identity, shutdown removing every interface, a forged
 overlay claim being rejected, and the core carrying the payload without
 interpreting it.
 
+`tests/local_control.rs` covers the local control socket end to end: a client
+asking a running agent for status over a real Unix socket, a leftover socket
+file being replaced while a live one is not, and the derived socket path
+staying short enough to bind.
+
 `tests/discovery.rs` covers the discovery contract itself: a static bootstrap
 candidate is enough to join, several backends compose, entries are withdrawn
 when a network stops, and a forgotten network stays forgotten across a restart.