Link container data regardless of kube talker - #395

As we link container data from an external process there is no need to check kube talker.

That is a bit complex, because adds parsing cgroup to all events. I have added guards to reduce the performance footprint for non-k8s use case. It should behave like this:

• An empty metadata tree causes no cgroup parse.
• A non-container cgroup is parsed only once.
• A container ID is built and allocated only once.
• A failed metadata lookup still occurs for each event while the tree is nonempty.
• A cgroup change clears the cached result.
• Container metadata that arrives later can still match the cached ID.

Implications:

• Initial metadata: all existing processes are linked in a full pass during a queue open.
• Later asynchronous metadata: no immediate full pass; processes are linked opportunistically when they next produce an event.
• A process that produces no subsequent event can remain unlinked until another linking pass or relevant event occurs.

Imho the last point is acceptable because no events meaning no messages to deliver from a queue.

TODO:

• Backend-less test fixture. Link container data regardless of kube talker #395 adds a shared quark_queue_init_trees helper plus its own stub queue ops in the C tests, but does not consolidate the two fixtures from Exposed OCI trees and helpers to go-quark #383 or the copy in the Go cgo preamble. There are now four copies instead of three.
• Explore alternative for dropping the quarktest tag. In this PR libquark exports quark_queue_init_trees in the header's "exported for testing only" section. With a one-line null guard on queue_ops in quark_queue_populate, matching the guard already in quark_queue_close, the stub ops become unnecessary. The Go helper then shrinks to a few lines that call calloc, call the init function, and set two fields, with no C preamble at all. It could sit unexported at the bottom of quark.go. That is test-only code in a production file, but it mirrors the precedent libquark already sets in its header. Precisely the same change also collapses the three hand-rolled C and Go fixtures, which is the fixture-duplication finding two items down the list.