Two-layer perf fix for the /api/reasoning storm dropping streaming to ~2 tok/s:
- ui.js: syncReasoningChip() serves the cached chip on routine topbar syncs, only
re-fetching when the model/provider key (_lastReasoningFetchKey) changes — restores
pre-a9ce2889 behavior while keeping its model-switch-refresh intent.
- config.py: memoize _load_yaml_config_file keyed on (path, st_mtime_ns, st_size) under
a lock; cache the RAW parsed dict and re-run cheap _expand_env_vars per call (cache stays
correct vs os.environ and read-modify-save callers get a fresh structure; on-disk edits
picked up via mtime/size). Code byte-faithful from PR head f868f10a1ad7. 9 own tests pass.
Co-authored-by: nesquena-hermes <agent@nesquena-hermes>
