Initial commit: DHCP whitelist service for direct link connections

Features:
- Docker-based DHCP server with MAC address whitelisting
- Binds to specific ethernet interface only
- NO DNS/gateway advertised (direct link only, not a router)
- Configurable network parameters (subnet, DHCP range, lease times)
- Systemd service integration for Arch/Manjaro
- Test environment with isolated network (172.20.0.0/24)
- Auto-configuration script to detect network settings
- Complete Makefile with management targets

Security:
- Only responds to whitelisted MAC addresses
- deny unknown-clients configuration
- Runs in Docker container for isolation

Configuration:
- Copy .example files to create your config
- interface.conf: Network interface to bind to
- whitelist.conf: Allowed MAC addresses
- network.conf: Network parameters (optional)

dhcp/dhcpd.conf.template

@@ -0,0 +1,18 @@
+authoritative;
+default-lease-time __LEASE_TIME__;
+max-lease-time __MAX_LEASE_TIME__;
+
+# Deny all clients by default
+deny unknown-clients;
+
+# Network configuration - Direct link only, no routing
+subnet __SUBNET__ netmask __NETMASK__ {
+    range __RANGE_START__ __RANGE_END__;
+    # No gateway - this is a direct link only
+    # No DNS - clients should use their own DNS from other interfaces
+    option subnet-mask __NETMASK__;
+    option broadcast-address __BROADCAST__;
+}
+
+# Whitelisted MAC addresses
+__HOST_ENTRIES__