chore: remove legacy deps (fnntw, rayon) and unused files

- Remove fnntw (KNN) and rayon dependencies, no longer needed with
  ensemble architecture
- Update burn features to include wgpu and train backends
- Remove dev.toml, models/.gitkeep, scripts/convert_csic.py, and
  pingora-headless.yaml (superseded by cluster gossip discovery)
- Add .DS_Store to .gitignore

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

.gitignore

@@ -4,8 +4,11 @@ certs/
 *.pem
 *.key
 *.crt
+*.tmp
+.DS_Store
 
 # Training data and model binaries
 *.bin
 *.jsonl
 heuristics.toml
+**/*artifacts