perf: enhance tokens block memory usage

[cheb0]

Description

Memory reduction (prod fraction):

message:*foobar*
25335032 => 20584912 bytes

message:*87466437*
58236912 => 71844952

Performance improvement: 5-10% for wildcards like message:*5647545* (cold requests). For wildcards with letters there is no benefit - we block on disk read time more since we skip blocks.

---

• 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 61.22449% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.04%. Comparing base (6372ee9) to head (c42261c).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
frac/sealed/token/block_loader.go	55.81%	15 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #486      +/-   ##
==========================================
- Coverage   71.12%   71.04%   -0.08%     
==========================================
  Files         232      232              
  Lines       18404    18533     +129     
==========================================
+ Hits        13089    13167      +78     
- Misses       4337     4383      +46     
- Partials      978      983       +5     

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