fix(server): fix stream stats corruption on topic purge

[hubcio]

Purging one topic while a sibling retained messages reported
zero messages for the entire stream via get_stats(). The
zero_out chain used store(0) at each tier and then called
zero_out on the parent, blanking the entire ancestor chain
rather than subtracting the purged child's contribution.

Replace store(0) with swap(0) + parent.decrement(prev) so
each tier only subtracts its own value. Remove the redundant
topic-level zero_out from purge_topic — partition zeroing
already propagates correctly through the hierarchy.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.55%. Comparing base (153cb37) to head (a7e2a20).

Files with missing lines	Patch %	Lines
core/server/src/streaming/stats/mod.rs	50.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2736      +/-   ##
============================================
- Coverage     68.58%   68.55%   -0.03%     
  Complexity      585      585              
============================================
  Files           735      735              
  Lines         60370    60342      -28     
  Branches      56783    56755      -28     
============================================
- Hits          41403    41366      -37     
- Misses        16941    16950       +9     
  Partials       2026     2026              

Flag	Coverage Δ
rust	70.08% <50.00%> (-0.04%)	⬇️

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/topics.rs	90.05% <ø> (+0.40%)	⬆️
core/server/src/streaming/stats/mod.rs	84.68% <50.00%> (-10.87%)	⬇️

... and 5 files with indirect coverage changes

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