circuit_send_intermediate_onion_skin() and extend_cell_format() should check for IPv6
When circuit_send_intermediate_onion_skin() and extend_cell_format() handle tor_addr_t, they assume they are IPv4.
But in legacy/trac#23502 (moved), we almost wrote code that sent them an IPv6 address. In this case, they put 0.0.0.0 in the extend cell, but they could issue a BUG() warning and refuse to send the cell instead.
Or they could send a proper IPv6 link specifier where the extend cell allows it.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information