Two distinct "config goes back to what it was after restart" bugs:

1) save_config dropped broker websocket_path + websocket_ping_interval (and the
   new offload gpu_split/tensor_split). The settings UI set them in memory, but
   save_config never serialized them, so the next load() fell back to dataclass
   defaults — the broker protocol/websocket settings reverted every boot even with
   a fully persistent config dir. Now serialized.

2) Per-model fields (e.g. n_ctx) reverted via a multi-process models.json write
   race: every engine loads models.json at its own boot, so a SECONDARY engine's
   in-memory models_data is stale w.r.t. a later UI edit on the primary. When that
   secondary engine auto-persisted measured_vram_gb it called save_models(), which
   dumps its whole stale state and clobbered the edit. Added
   ConfigManager.persist_model_field(): a single-field, atomic read-modify-write
   that re-reads models.json from disk, updates only the one field, and refreshes
   the in-memory copy. record_vram_delta now uses it, so VRAM measurement can never
   revert user model config.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>