feat: make fsync configurable via fraction config

[dmedovich]

Fixes #199

Added SkipFsync bool to frac.Config and common.SealParams.
So fsync can be disabled at the fraction level, not just globally via resources.skip_fsync.
Useful in unit tests — setting frac.Config{SkipFsync: true} cuts test time roughly in half when running hundreds of tests. Global resources.skip_fsync still works and now propagates into fraction config automatically on startup.

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[cheb0]

overall looks good to me. let's enable skip fsync feature in fraction_test.go

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.38095% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.46%. Comparing base (b4b9f19) to head (6d9505f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
frac/active_sealing_source.go	42.85%	2 Missing and 2 partials ⚠️
cmd/seq-db/seq-db.go	0.00%	2 Missing ⚠️
frac/active.go	71.42%	2 Missing ⚠️
frac/sealed/sealing/sealer.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #417      +/-   ##
==========================================
- Coverage   71.46%   71.46%   -0.01%     
==========================================
  Files         219      219              
  Lines       16448    16455       +7     
==========================================
+ Hits        11755    11759       +4     
- Misses       3820     3824       +4     
+ Partials      873      872       -1     

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