Monitor v0.7.0 serialization downgrade compatibility

Add a downgrade canary that writes current node state through the legacy
v1 filesystem store and reopens it with ldk-node v0.7.0. This monitors
whether serialized node, channel, and payment state remains usable by
v0.7.0, including a restored channel and a post-restart payment.

This does not assert that the current filesystem-store v2 IO layout can
downgrade to v0.7.0's v1 layout. That IO-layer downgrade is unsupported:
v2 stores empty namespaces under [empty], which v1 readers do not look
up.

Co-Authored-By: HAL 9000

Author: tnull