[AhmetNSimsek]

This PR makes configuration loading independent of the directory layout while preserving the existing folder-based fast path.

Configuration.build_objects() now accepts an optional expected class. If the configured folder exists, objects are loaded exactly as before. If it is missing, the loader falls back to all discovered configuration specs and keeps only objects matching the requested class.

Related changes:

• remove folder-existence checks from concept registries and feature loading;
• rely on RepositoryConnector for file discovery and keep Configuration responsible for selecting relevant specs;
• keep Factory responsible solely for constructing objects;
• add type hints for configuration loaders and build_objects();
• avoid maintaining a separate global loader index by flattening the existing folder-indexed loaders only on the fallback path.

This keeps standard siibra configurations fast while allowing valid configuration JSON files to be placed in arbitrary directories.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.20%. Comparing base (3036233) to head (8a328f9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
siibra/configuration/configuration.py	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #725      +/-   ##
==========================================
- Coverage   46.21%   46.20%   -0.01%     
==========================================
  Files          74       74              
  Lines        8393     8390       -3     
==========================================
- Hits         3879     3877       -2     
+ Misses       4514     4513       -1     

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