Per-connection child launcher for the partition supervisor.

nquic_server_sup (the simple_one_for_one partition supervisor) has a single, fixed child MFA. This launcher is that child: given the per-connection option map it starts the right owner.

• Default (no conn_handler): nquic_conn_statem, the handshake gen_statem that exports the connection to a nquic:accept/2 caller.
• With conn_handler => Module: Module, which owns the connection from the first packet and drives the handshake itself via nquic_lib:server_accept_init/1 (owner-from-first-packet, no export, accept queue, or takeover). Module must export start_link/1 taking the option map and returning {ok, pid()}; the returned pid is the connection owner the partition supervisor links to.

The launcher returns the started owner's pid directly, so the partition supervisor links/monitors the owner, not the launcher. Children are temporary, so the recorded launcher MFA is never re-invoked.