perf: improve compression throughput and reduce allocations

[eguguchkin]

Parallelize compression and optimize memory allocations

• Parallelize docs and metas compression in DocsMetasCompressor using goroutines
• Pre-allocate 1MiB capacity for binaryDocsPool and binaryMetasPool to reduce allocations
• Skip zero-value metric increments in MergeQPRs when repetitionsCount is 0
• Fix partial indexing logic in keyword/path/text tokenizers: now correctly truncating oversized values instead of always discarding the tail without partial indexing

---

• I have read and followed all requirements in CONTRIBUTING.md;
• I used LLM/AI assistance to make this pull request;

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.48%. Comparing base (124d460) to head (bce6fea).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
+ Coverage   71.40%   71.48%   +0.08%     
==========================================
  Files         219      219              
  Lines       16454    16460       +6     
==========================================
+ Hits        11749    11767      +18     
+ Misses       3834     3821      -13     
- Partials      871      872       +1     

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[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
Click on Show table button to see full list of degraded benchmarks.

Show table

Name	Previous	Current	Ratio	Verdict
ProcessDocuments-4	124d46	fa9305
	4.00 allocs/op	7.00 allocs/op	1.75	🔴