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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• I used LLM/AI assistance to make this pull request;

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.29%. Comparing base (a73114c) to head (ee4ca9c).

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   71.33%   71.29%   -0.04%     
==========================================
  Files         233      233              
  Lines       18999    18998       -1     
==========================================
- Hits        13552    13545       -7     
- Misses       4419     4427       +8     
+ Partials     1028     1026       -2     

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[dkharms]

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

LGTM
Thank you for contribution!

[cheb0]

@dkharms Looks good to me. We will have a version bump in 0.77, attached to the corresponding milestone