Summary

• extract direct session subscriber state and filtered fan-out into SessionSubscriberManager
• expose session phase transition legality through a focused helper and align Passivating transitions with the spec
• add focused tests for subscriber filtering/snapshots and phase transition rules

Validation

• dotnet test src/Netclaw.Actors.Tests --filter "FullyQualifiedNameSessionPhaseTransitionTests|FullyQualifiedNameSessionSubscriberManagerTests"
• dotnet test src/Netclaw.Actors.Tests --filter "FullyQualifiedName~LlmSessionIntegrationTests"
• dotnet test src/Netclaw.Actors.Tests --filter "FullyQualifiedName~SubAgentSpawnIntegrationTests"
• dotnet test
• dotnet slopwatch analyze
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• git diff --check