Fix FileFeatureStorage instrument directory casing

[Jaco-Ren]

Summary

• resolve FileFeatureStorage feature paths against existing instrument directories so vendor-preserved casing such as ABB-U is honored on case-sensitive filesystems
• keep the historical lowercase path as the fallback when no matching directory exists
• add a regression test that reads a feature file from features/ABB-U/close.day.bin

Motivation and Context

Fixes #2053.

FileFeatureStorage previously constructed feature paths with instrument.lower(), which works for qlib's lowercase datasets but fails on case-sensitive filesystems when local/vendor data is stored with uppercase or mixed-case instrument directories. In that case every feature request resolves to the lowercased path and returns empty data even though the requested instrument's files exist.

Testing

• python -m py_compile qlib\data\storage\file_storage.py tests\storage_tests\test_storage.py
• git diff --check
• isolated FileFeatureStorage harness verifying ABB-U resolves to the on-disk features/ABB-U directory and reads the expected values

I also attempted python -m pytest tests/storage_tests/test_storage.py::TestFileFeatureStorageCaseResolution -q, but this local Windows source checkout cannot collect qlib tests until qlib.data._libs.rolling is compiled. python setup.py build_ext --inplace is blocked locally by missing Microsoft Visual C++ 14.0+ Build Tools.

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