refactor: remove ID count from offsets

[spacer07]

Description

This completes the binary format cleanup started in #410. It:

• bumps the fraction binary data version to V6;
• stops writing the legacy ID count placeholder to the offsets section;
• keeps V0–V5 fractions readable by skipping the legacy field for old versions;

Fixes #409

---

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.10%. Comparing base (3f9dedd) to head (1a5b840).

Files with missing lines	Patch %	Lines
frac/sealed_loader.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #478      +/-   ##
==========================================
+ Coverage   71.07%   71.10%   +0.03%     
==========================================
  Files         232      232              
  Lines       18405    18404       -1     
==========================================
+ Hits        13082    13087       +5     
+ Misses       4342     4339       -3     
+ Partials      981      978       -3     

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[dkharms]

@cheb0 can you take a look as well, please?

LGTM
Thank you for contribution!