fix(server): memory leak in segment rotation

[spetz]

Segment rotation accumulated memory indefinitely because sealed segments retained their 16MB index buffers and kept file writers open. Under heavy write load with frequent rotations, this caused memory to balloon from ~100MB to 20GB+.

The fix clears index buffers for sealed segments (when cache_indexes != All) and closes their writers immediately after sealing. Writers are never needed post-seal, and this also releases io_uring/kernel file handle resources.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.78%. Comparing base (ff2da26) to head (2f67133).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2686      +/-   ##
==========================================
+ Coverage   69.69%   69.78%   +0.09%     
==========================================
  Files         568      568              
  Lines       55229    55238       +9     
  Branches    55229    55238       +9     
==========================================
+ Hits        38492    38550      +58     
+ Misses      14859    14809      -50     
- Partials     1878     1879       +1     

Flag	Coverage Δ
rust	69.78% <100.00%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
core/server/src/shard/system/segments.rs	86.66% <100.00%> (+8.09%)	⬆️

... and 7 files with indirect coverage changes

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[hubcio]

Nice. Please modify the segment rotation scenario to include probing for memory used by iggy-server when test ends so we will avoid this problem in future. Set it to some arbitrary value like 200MB.