refactor: do not store ids count in offsets

[dkharms]

Description

Previously, BlockOffsets stored the total number of document IDs alongside the block offsets. This meant WriteOffsetsFile had to read from Info, which is a problem for compaction — we don't have complete Info while building the index.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.63%. Comparing base (a398a4e) to head (dfa67ba).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
frac/sealed_loader.go	33.33%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #410      +/-   ##
==========================================
- Coverage   70.65%   70.63%   -0.02%     
==========================================
  Files         226      226              
  Lines       17614    17611       -3     
==========================================
- Hits        12445    12440       -5     
  Misses       4245     4245              
- Partials      924      926       +2     

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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.

Show table

Name	Previous	Current	Ratio	Verdict
ProcessDocuments-4	3fd6f8	a7f42d
	508.00 B/op	11620.00 B/op	22.87	🔴
Sealing_NoSort-4	3fd6f8	a7f42d
	24879096.00 B/op	385124584.00 B/op	15.48	🔴
Sealing_WithSort-4	3fd6f8	a7f42d
	40310296.00 B/op	400500424.00 B/op	9.94	🔴