fix(connect): preserve tunnel delivery and late wrappers

[minekube-ai-engineer]

Summary

• flush each accepted tunnel packet atomically inside one FIFO event-loop task
• resolve Velocity's current frontend initializer for every tunnel instead of retaining the startup snapshot
• preserve the Connect tunnel chat-session filter after third-party wrappers initialize the channel

Reproduction: delayed keepalive delivery (#140)

The new TunnelHandlerTest.onReceiveFlushesBeforeTheNextEventLoopTaskCanObserveDelivery inserts an observation task immediately after packet acceptance. The previous two-task batching path produced [WRITE, OBSERVE, FLUSH], proving unrelated event-loop work could run before the accepted packet was delivered. The fixed path produces [WRITE, FLUSH, OBSERVE] and keeps payload copying and event-loop confinement.

The later Windows socket abort in the report is cleanup after Paper's kick, not the initiating cause. An affected-host 0.15.4 retest is still required before closing #140.

Reproduction: PacketEvents/nLogin (#141)

The Velocity regression test installs a PacketEvents-shaped wrapper after Connect creates its tunnel initializer. The previous implementation invoked only the captured Velocity initializer and omitted the later decoder. The fixed path resolves the holder per connection, so both the base Velocity decoder and late PacketEvents decoder are present.

Validation

• focused RED: delayed-flush test returned [WRITE, OBSERVE, FLUSH]
• focused GREEN: complete TunnelHandlerTest passed
• focused Velocity late-binding test passed
• full ./gradlew check passed

Closes #141
Addresses #140