feat: store .remote-info for remote shards to avoid extra S3

[eguguchkin]

Fixes #435

---

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

[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
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Name	Previous	Current	Ratio	Verdict
AggWide/size=1000000-4	8fb975	01db29
	469.00 B/op	545.00 B/op	1.16	🔴
MutexListAppend-4	8fb975	01db29
	215.03 MB/s	193.50 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.

Show table

Name	Previous	Current	Ratio	Verdict
MutexListAppend-4	8fb975	a8fa9b
	215.03 MB/s	192.22 MB/s	0.89	🔴

[codecov-commenter]

Codecov Report

❌ Patch coverage is 58.24742% with 81 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.17%. Comparing base (7cc2790) to head (c89ba52).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
frac/sealed.go	43.47%	26 Missing and 13 partials ⚠️
frac/remote.go	61.66%	14 Missing and 9 partials ⚠️
util/fs.go	46.66%	4 Missing and 4 partials ⚠️
frac/sealed_source.go	66.66%	4 Missing and 2 partials ⚠️
fracmanager/frac_manifest.go	72.22%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #436      +/-   ##
==========================================
+ Coverage   71.16%   71.17%   +0.01%     
==========================================
  Files         232      232              
  Lines       18466    18546      +80     
==========================================
+ Hits        13141    13201      +60     
- Misses       4343     4351       +8     
- Partials      982      994      +12     

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[dkharms]

By the way, we can get rid of this awkward return signature (bool, error) and just return error because fractionProvider will always try to offload sealed fractions.

If you remember, I've added this boolean to distinguish between offloading attempts of active and sealed fractions.

[eguguchkin]

done

[dkharms]

Seems like if seq-db crashes (for some reason) in the middle of util.WriteFileAtomic and we end up with seq-db-*.remote-info.tmp -- it won't be able to boot.

I guess it's better to just perform rename from .info to .remote-info, right?