feat: add RFC 3927 support (#114)

Signed-off-by: Marvin Drees <marvin.drees@9elements.com>

README.md

@@ -100,6 +100,7 @@ ok      github.com/soypat/lneto/x/xnet  2.926s
 | TCP | RFC 9293 | ✅ | `tcp` | 0 ²³ | Full state machine, SYN cookies, retransmit queue, `Conn`/`Listener` |
 | DNS | RFC 1035 | ✅ | `dns` | — | Client (A/AAAA query) |
 | DHCPv4 | RFC 2131 | ✅ | `dhcp/dhcpv4` | — | Client + Server |
+| IPv4 Link-Local (APIPA) | RFC 3927 | ✅ | `ipv4/linklocal4` | 0 | Heapless claim-and-defend state machine for 169.254.x.x |
 | NTP | RFC 5905 | ✅ | `ntp` | — | Client |
 | NTS | RFC 8915 | ✅ | `x/nts` | — | Client + Server; key exchange over TLS 1.3, authenticated NTP. Requires caller-supplied AEAD_AES_SIV_CMAC_256 |
 | mDNS | RFC 6762 | ✅ | `dns/mdns` | — | Client (service announcement + query) |
@@ -122,6 +123,7 @@ ok      github.com/soypat/lneto/x/xnet  2.926s
     - [`lneto/internet/pcap`](./internal/pcap): Packet capture and field breakdown utilities. Wireshark in the making.
 - [`lneto/http/httpraw`](./http/httpraw/): Heapless HTTP header processing and validation. Does no implement header normalization.
 - [`lneto/tcp`](./tcp): TCP implementation and low level logic.
+- [`lneto/ipv4/linklocal4`](./ipv4/linklocal4): RFC 3927 IPv4 link-local (APIPA) address autoconfiguration. Heapless claim-and-defend state machine for self-assigned 169.254.x.x addresses when no DHCP server is available.
 - [`lneto/dhcpv4`](./dhcpv4): DHCP version 4 protocol implementation and low level logic.
 - [`lneto/dns`](./dns): DNS protocol implementation and low level logic.
 - [`lneto/ntp`](./ntp): NTP implementation and low level logic. Includes NTP time primitives manipulation and conversion to Go native types.