feat(net): real UDP transport, BSD datagram sockets, DNS resolver, DHCP client

Turn the networking stubs into a working transport/application stack on top of
the existing L2/L3 (Ethernet/ARP/IPv4/ICMP) core.

UDP (net/ipv4/udp.c)
  - Port-demux table with bound receive handlers, pseudo-header checksum,
    ephemeral-port allocation, limited-broadcast send (0.0.0.0 -> 255.255.255.255
    for DHCP before the interface has an address), and rx/tx/drop counters.
  - Wired into the IPv4 receive path (IP_PROTO_UDP dispatch in net/core/net.c).

BSD sockets (net/core/socket.c)
  - Real SOCK_DGRAM sockets backed by UDP: bind installs a UDP handler that fills
    a per-socket receive ring; send/sendto and recv/recvfrom move datagrams;
    ephemeral auto-bind, connect() memorises the peer. Socket table + I/O stats
    exposed for introspection.

DNS resolver (net/ipv4/dns.c)
  - Builds and parses real A-record queries over UDP/53 (label encoding,
    compression-pointer aware answer walk), cache-first with TTL, static-hosts
    and dotted-quad fast paths, and a bounded timeout that degrades to fallback
    instead of hanging.

DHCP client (net/ipv4/dhcp.c)
  - Full DISCOVER/OFFER/REQUEST/ACK handshake over UDP broadcast, option parsing
    (mask/router/dns/lease/server-id), applies the lease to the primary
    interface, bounded timeouts keep the static config on no response.

Fix (net/ipv4/dns_cache.c): the cache stored IPs as 0xAABBCCDD numeric while the
rest of the stack uses network-byte-order-in-memory (ip_parse/ip_to_str), so a
resolved address came back byte-swapped (nslookup github.com -> 100.121.82.140).
Seeds and dump now use ip_parse/ip_to_str; verified 140.82.121.4.

Userland (lazybox): live ifconfig (real interface + NIC counters, loopback),
netstat with live UDP bindings and transport stats, new `sockstat`, richer
nslookup. Verified in QEMU: boot clean on x86_64/aarch64/armv8i, login as SUB,
ping/nslookup/dhclient/netstat/sockstat all run; DHCP/DNS wire replies are
blocked by the sandbox's slirp but time out cleanly and fall back as designed.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

include/net/socket.h

@@ -41,6 +41,18 @@ int sys_bind(int sockfd, const struct sockaddr_in* addr, size_t addrlen);
 int sys_connect(int sockfd, const struct sockaddr_in* addr, size_t addrlen);
 ssize_t sys_send(int sockfd, const void* buf, size_t len, int flags);
 ssize_t sys_recv(int sockfd, void* buf, size_t len, int flags);
+
+// Address-carrying variants for connectionless (SOCK_DGRAM) use.
+ssize_t sys_sendto(int sockfd, const void* buf, size_t len, int flags,
+                   const struct sockaddr_in* dest, size_t addrlen);
+ssize_t sys_recvfrom(int sockfd, void* buf, size_t len, int flags,
+                     struct sockaddr_in* src, size_t* addrlen);
+
 int sys_close_socket(int sockfd);
 
+// Introspection for netstat / sockstat.
+int  socket_get_count(void);
+const socket_t* socket_get(int idx);
+void socket_get_stats(uint64_t* tx_dgrams, uint64_t* rx_dgrams, uint64_t* drops);
+
 #endif // _NET_SOCKET_H