FileFeatureStorage lowercases the instrument directory name when building feature paths

FileFeatureStorage lowercases the instrument directory name when building feature paths (instrument.lower()/field.lower().freq.bin). On case‑sensitive filesystems (e.g., Linux) where the vendor data is stored with uppercase instrument folders (data/xsto/features/ABB-U/...), the loader looks for abb-u/…, can’t find the file, and every expression resolves to an empty dataframe even though the data exists.

To Reproduce

Steps to reproduce the behavior:

Store local feature data under uppercase instrument directories (e.g., data/xsto/features/ABB-U/close.day.bin).
Initialize qlib with that provider URI.
Request any feature for that instrument, e.g. D.features(["ABB-U"], ["$close"], "2000-01-01", "2025-12-31"). The result is an empty dataframe and the feature storage backend reports the file is missing.

Expected Behavior

FileFeatureStorage should respect the actual on-disk casing instrument list (or probe common case variants) so that features load correctly regardless of filesystem case sensitivity.

Screenshot

N/A – the console logs show statements such as [FileFeatureStorage.getitem] uri=…/features/abb-u/close.day.bin missing for request=… even though …/features/ABB-U/close.day.bin exists.

Environment

Version: 0.9.7
Python version: 3.12 (virtual env)
OS: Linux (Ubuntu on AMD cloud server, case-sensitive filesystem)
Commit number: N/A (using pip wheel)
Additional Notes
We patched FileFeatureStorage locally to keep multiple filename candidates (instrument.lower(), original case, uppercase) and select the first existing path before reading. After this change ABB-U and other uppercase symbols load correctly. Happy to submit a PR if this approach sounds acceptable.

Locate FileFeatureStorage and trace how instrument and field names become feature paths. Reproduce with data/xsto/features/ABB-U/close.day.bin and a D.features request for ABB-U/$close; the fix is complete when the existing uppercase file loads on a case-sensitive filesystem instead of returning an empty dataframe.