Add ping tool as a separately-releasable application module

[JohnAmadis]

Summary

• Adds ping, a CLI to send ICMPv4 Echo Requests and report replies, under tools/ping - a command in dmicmp's new tools/ directory, following the same pattern dmnet/tools/ifconfig and dmnet/tools/ip already use in the ecosystem.
• ping is its own Application-type DMOD module (own CMakeLists.txt + ping.dmr), so every tagged release of this repo now also packages ping-<tag>-<arch>.zip alongside dmicmp itself - no change to .github/workflows/release.yml was needed, since it already discovers and packages every module under build/packages/*.
• ping is a thin CLI over dmicmp's existing Echo Request/Reply API (dmicmp_v4_send_echo_request(), dmicmp_register_echo_listener()/_unregister_echo_listener()) - it builds/parses no ICMP bytes itself. It links dmicmp's own dmicmp_if target directly (a same-repo sibling module, like tests/CMakeLists.txt already does) rather than fetching dmicmp's headers back from GitHub as if it were external.
• Supports -c <count>, -i <interval_ms>, -W <timeout_ms>, -s <size>, and requires a literal IPv4 address (no hostname resolution, no ICMPv6 send path yet - same limitation dmicmp itself documents).
• Adds tools/ping/tests/ping_test.c, driven through Dmod_RunModule("ping", ...) the same way ifconfig_test.c drives ifconfig, plus docs/ and a README.md for the new module.

Test plan

• cmake configure succeeds with a local dmod checkout and correctly wires the ping/test_ping targets (verified locally; this sandbox has no dmf-get/network access to run a full build or ctest, same as CI's containerized build)
• tools/ping/src/ping.c and tools/ping/tests/ping_test.c compile cleanly (gcc -fsyntax-only -Wall -Wextra) against the real dmicmp/dmip/dmroute/dmnetif/dmarp/dmosi headers from their published repos
• CI (.github/workflows/ci.yml) builds dmicmp + ping and runs test_dmicmp/test_ping via dmod_loader

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https://claude.ai/code/session_01V4i6Vs6fjG9cdHGj3AMpD5

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