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 70.35%. Comparing base (a79eceb) to head (7ea2f82).
⚠️ Report is 1 commits behind head on main.

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   70.53%   70.35%   -0.18%     
==========================================
  Files         232      232              
  Lines       18319    18328       +9     
==========================================
- Hits        12921    12895      -26     
- Misses       4420     4464      +44     
+ Partials      978      969       -9     

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[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
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Name	Previous	Current	Ratio	Verdict
MutexListAppend-4	25381c	8f3748
	199.21 MB/s	179.00 MB/s	0.90	🔴

[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
Block_Unpack-4	4ad8f1	78a38d
	90901.00 ns/op	108792.00 ns/op	1.20	🔴