chore(refactor): collapse conntrack into packetparser

[nddq]

Description

pkg/plugin/conntrack/ lived under pkg/plugin/ but was never actually a plugin — it didn't implement the plugin interface, didn't self-register in include_linux.go, and its only real consumers were packetparser (which #included conntrack's C code directly) and a GC loop that pluginmanager launched iff packetparser was enabled. The arrangement obscured the fact that conntrack is a component of packetparser.

This PR collapses conntrack into the packetparser package so the relationship is explicit.

• C: Merge _cprog/conntrack.c into _cprog/conntrack.h and move it into packetparser's _cprog/. packetparser.c now includes conntrack.h once (previously included both .c and .h). Drop the separate bpf2go target — packetparser.o owns the retina_conntrack map via LIBBPF_PIN_BY_NAME.
• Go: Move the userspace GC loop into pkg/plugin/packetparser/conntrack_linux.go as a free function. packetParser.Start() launches it; it operates on p.objs.RetinaConntrack directly (no more ebpf.LoadPinnedMap).
• Init path: Add packetparser.InitConntrackMap() for the init container to pre-create and pin the map from packetparser's generated spec. The agent container isn't privileged enough to pin new maps, so the pin must already exist when packetparser.o is loaded in the agent container.
• Pluginmanager: Drop the pluginNamePacketparser special case and the associated conntrack.New/Run block — packetparser owns its own lifecycle now.
• Tests: packetparser_ebpf_test.go drops the separate conntrack dynamic.h write and the -I../conntrack/_cprog/ include.

Related Issue

N/A

Checklist

• I have read the contributing documentation.
• I signed and signed-off the commits (git commit -S -s ...). See this documentation on signing commits.
• I have correctly attributed the author(s) of the code.
• I have tested the changes locally.
• I have followed the project's style guidelines.
• I have updated the documentation, if necessary (no user-facing changes).
• I have added tests, if applicable (pure refactor — existing tests cover behavior).

Screenshots (if applicable) or Testing Completed

Additional Notes