feat(transport): graceful session migration on tunnel failure

[Yanstart]

Context

Standards alignment: RFC 3135 §4 (PEP failure modes).

When the QUIC tunnel fails (satellite link loss, handover failure), migrate active TCP sessions to a new tunnel connection without dropping client connections.

Requirements

• Buffer in-flight data during tunnel reconnection
• Resume TCP streams on new QUIC connection
• Configurable timeout before session teardown
• Integrate with event system (arch: event system design for satellite events (#5 foundation) #75) for proactive migration

References

• RFC 3135 §4
• NTN conformance: docs/standards/NTN-CONFORMANCE.md §1.1