refactor(storage): load index registry once in IndexReader constructor

[eguguchkin]

Description

The index registry (the block-header table of an index file) was previously loaded lazily through a cache.Cache[[]byte], so every BlocksCount() / GetBlockHeader() call took a cache lookup guarded by a mutex. On the frac-load path these calls are frequent and concurrent across readers, and mutex contention on the shared cache entry made them measurably expensive.

This PR loads the registry once in NewIndexReader into a plain registry []byte field. Since the registry is immutable for the lifetime of the reader, reads are now lock-free — the mutex is paid once per reader instead of on every access.

As a side effect, BlocksCount() becomes infallible (int instead of (int, error)), so error handling disappears from frac/sealed_loader.go and frac/sealed/token/table_loader.go.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.21%. Comparing base (961ed42) to head (f2cd118).

Files with missing lines	Patch %	Lines
storage/index_reader.go	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #568      +/-   ##
==========================================
- Coverage   79.21%   79.21%   -0.01%     
==========================================
  Files         249      249              
  Lines       18844    18828      -16     
==========================================
- Hits        14927    14914      -13     
+ Misses       3917     3914       -3     

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