Change activeTCPConn.close to atomic.Bool

[cppcoffee]

Replace manual atomic operations with atomic.Bool type for better type safety and cleaner code. This modernizes the atomic usage pattern from atomic.LoadInt32/StoreInt32 to the newer Load/Store methods on atomic.Bool.

• Update activeTCPConn.closed field type from int32 to atomic.Bool
• Replace atomic.LoadInt32(&a.closed) with a.closed.Load()
• Replace atomic.StoreInt32(&a.closed, 1) with a.closed.Store(true)

All existing functionality preserved with improved type safety.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.55%. Comparing base (753c2a0) to head (ae256b8).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
active_tcp.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #782      +/-   ##
==========================================
+ Coverage   78.38%   78.55%   +0.16%     
==========================================
  Files          41       41              
  Lines        5441     5441              
==========================================
+ Hits         4265     4274       +9     
+ Misses        945      937       -8     
+ Partials      231      230       -1     

Flag	Coverage Δ
go	78.55% <85.71%> (+0.16%)	⬆️
wasm	27.08% <0.00%> (ø)

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[cppcoffee]

Changed to atomic.Bool type.