Claim ICMP protocols via dmip's protocol handler DIF

[JohnAmadis]

Summary

• Companion to choco-technologies/dmip#3, which removes dmip_register_protocol()/_register_default_protocol() in favor of a DIF dmip itself defines.
• dmicmp now implements dmip's protocol handler DIF instead, claiming DMIP_PROTO_ICMP, DMIP_PROTO_ICMPV6, and the new DMIP_PROTO_DEFAULT fallback marker all from one dmip_protocol_numbers() implementation.
• Since a DIF implementation is one function per module rather than one callback per registration, the previous two separate handlers (dmicmp_handle_ip_packet() for ICMP/ICMPv6, dmicmp_handle_unclaimed_protocol() for the default fallback) are merged into a single dmip_protocol_receive() implementation that re-parses the packet's own protocol field to tell the two cases apart.
• No dmod_init()/_deinit() registration call is needed anymore - dmicmp can now be reloaded without dmip holding a reference on it.
• CMakeLists.txt now declares DMOD_DIF_IMPLS dmip.

Test plan

• Verified src/dmicmp.c and include/dmicmp.h compile cleanly (gcc -fsyntax-only) against real dmod core headers, the updated dmip headers (from the companion PR), and real peer-module headers, since dmf-get/network package fetch wasn't available in this environment to run the full CMake+ctest flow.
• Confirmed tests/dmicmp_test.c needs no changes (it drives the module's real DIF implementation through dmip's real dispatch, the same way it already exercised the old registration) and still compiles against the new headers.
• CI (ctest via dmod_loader) on this PR, once merged alongside Replace protocol registration with dmip's own DIF dmip#3.

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