perf(search): replace map with slice to improve histogram performance

[eguguchkin]

replace map with slice to improve performance for billion-access queries

Fixes #170

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• I used LLM/AI assistance to make this pull request;

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.36364% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.58%. Comparing base (abd6bfb) to head (480019b).

Files with missing lines	Patch %	Lines
t.go	0.00%	11 Missing ⚠️
frac/processor/search.go	79.31%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #171      +/-   ##
==========================================
+ Coverage   71.37%   71.58%   +0.21%     
==========================================
  Files         200      201       +1     
  Lines       18191    18227      +36     
==========================================
+ Hits        12984    13048      +64     
+ Misses       4492     4463      -29     
- Partials      715      716       +1     

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