feat: migrate audit server runtime to gnet v2

[ncode]

Summary

• migrate audit server networking integration from gnet v1 to gnet v2 by switching to gnet.Run, embedding gnet.BuiltinEventEngine, and adding an OnTraffic adapter that delegates to a shared frame handler
• preserve current behavior and test compatibility by keeping React as a temporary shim while removing v1 dependency from go.mod and updating gnet imports/tests
• capture and archive the completed OpenSpec change (migrate-gnet-v2), sync main spec for gnet-v2-runtime-compatibility, and seed a follow-up change for React-to-OnTraffic test migration

Verification

• go test -v ./pkg/auditserver -run 'TestAuditServer_React|TestReact_.*'
• go test -v ./cmd -run 'TestAuditServerCmd_.*'
• go test -v ./pkg/auditserver -run '^$' -bench '^BenchmarkReact$' -benchtime=1x
• go test -tags=integration -run '^$' ./...
• go fmt ./...
• go vet ./...
• go test -v -race ./pkg/auditserver ./cmd
• go test -v -race ./...

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.64%. Comparing base (5ce6478) to head (1ca1714).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   95.39%   95.64%   +0.24%     
==========================================
  Files          10       10              
  Lines         543      551       +8     
==========================================
+ Hits          518      527       +9     
+ Misses         20       19       -1     
  Partials        5        5              

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