feat: add token block size config

[cheb0]

Description

Not for performance or compression reasons, but mostly for tests. Smaller blocks can catch more edge cases.

---

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.52%. Comparing base (25381c2) to head (862caf8).

Files with missing lines	Patch %	Lines
cmd/seq-db/seq-db.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #439   +/-   ##
=======================================
  Coverage   70.51%   70.52%           
=======================================
  Files         232      232           
  Lines       18268    18277    +9     
=======================================
+ Hits        12881    12889    +8     
- Misses       4426     4427    +1     
  Partials      961      961           

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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.

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Name	Previous	Current	Ratio	Verdict
MutexListAppend-4	25381c	8f3748
	199.21 MB/s	179.00 MB/s	0.90	🔴