#1856 Host memory probe: accept the root cgroup v2 group ("0::/", WSL without systemd)

Description

## Title
Issue: none open, found it on my own setup.

## Summary
Ran into this on WSL2 (Ubuntu 22.04, no systemd): `/proc/self/cgroup` is just `0::/`, so the path ends up as `/sys/fs/cgroup/` with a trailing slash. That never matches the root, and since the root group has no `memory.max` the probe gives up, so the engine doesn't know how much RAM is free.

## What changed
- `src/core/expert_source.cpp`: use the root directly when the line is `0::/`.
- `tests/core/file_expert_source_test.cpp`: added a test for it, it fails without the fix and passes with it.

## Extra Notes
- Been running 0.1.31 with this patch on a 3090 + IQ2_XS with `--resident-experts` for a while now without issues.
- Also it's better to copy the model files in your wsl disk rather than accessing it from /mnt/ as WSL-Windows Bridge is super slow (top at ~90 mb / sec instead of 3-4 GB/s)