runtime: pb.from_pb — build modules from a binary FileDescriptorSet

Complements pb.parse (which consumes .proto source) by accepting the
output of `protoc --descriptor_set_out=...`. Returns
{files={[name]=module}, order, lookup} where each per-file module has
the same shape as pb.parse() output.

Pipeline: hand-built descriptor.proto descriptors in descriptor_pb.lua
decode the wire bytes via pb.codec; fileset.lua translates each
FileDescriptorProto into the AST shape pb.parser emits;
pb.dynamic.build consumes the AST.

Handles map-entry reconstruction (synthetic entry messages skipped from
nested_messages, key/value lifted to the AST map field), proto3_optional
rehydrated as optional=true rather than a synthetic oneof, oneof
grouping, nested types, WKT references.

Tests parity against the statically-generated hello module — 17 cases
covering scalars/repeated/optional/oneof/maps/self-reference/enums/WKTs
plus a full round-trip. 403/403 luatest green.