perf: find TokenTable index once

[dkharms]

Description

TableLoader.Load previously re-scanned all token blocks on every call to find the start of the token table. This commit extracts that scan into advanceToTable, guarded by a sync.Once, so the linear scan over token blocks happens at most once per TableLoader instance.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.63%. Comparing base (e0cf437) to head (bc14b51).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
- Coverage   70.65%   70.63%   -0.03%     
==========================================
  Files         219      219              
  Lines       16967    16968       +1     
==========================================
- Hits        11988    11985       -3     
- Misses       4084     4086       +2     
- Partials      895      897       +2     

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