kernel stack traces

Once we have #13 we can add kernel backtraces to profile/kprobe runs.

- [ ] capture kernel stack trace using `bpf_get_stack`
- [ ] symbolize kernel stack trace using `/proc/kallsyms`
- [ ] add kernel parts to flamegraph

Start by reviewing issue #13 and the existing profile/kprobe path. Trace how stack data reaches the flamegraph, then investigate the three checklist items: capturing with bpf_get_stack, symbolizing through /proc/kallsyms, and including kernel frames in the flamegraph. Done means all three items work for profile/kprobe runs and are covered by the project's relevant checks.

Written by the indexing model from the issue text.