Adds a tree/mmp-targeted compose overlay (compose-trace-tree.yml) and
a new FIPS_MESH_LAB_TRACE_TREE env-var gate in run_rekey_family,
layered independently of the existing FIPS_MESH_LAB_TRACE rekey-class
overlay. Trace targets: fips::node::tree, fips::tree,
fips::node::handlers::mmp, fips::node::handlers::handshake.
Used for tree-partition race investigations during multi-peer startup
where evaluate_parent inputs, send_tree_announce_to_all recipient
enumeration, and process_receiver_report first-RTT triggers all need
to be visible together to bracket the loss window.
