Runtime loader for Reflow actor packs.
A pack is a cdylib that exports two symbols:
reflow_pack_abi_version(): u32— handshake againstREFLOW_PACK_ABI_VERSIONreflow_pack_register(host: *mut PackHostVtable): i32— calls back into the host to register template factories
Packs can be loaded either as raw dylibs (developer loop) or wrapped in
a .rflpack zip bundle (distribution format). See [bundle].