flutter3d_hub 1.0.0-rc.1

The model hub's client contract for flutter3d: the reference format `<uuid>@<n>`, the refusals, the cache policy and the versioned server protocol. Plain Dart with no engine behind it.

1.0.0-rc.1

• New: the model hub's client contract, as a package of its own. HubReference reads the reference format <uuid>@<n> with an optional #sha256= checksum, fixed for good, through one function, parse, that refuses anything else with a HubReferenceFormatException saying what is wrong. HubRefusedException carries a HubRefusal (notFound, withdrawn, hashMismatch, offline, tooLarge, protocol), an open set. HubCachePolicy says where a model may come from and whether it is kept.
• New: the server protocol as an interface. HubProtocol.version is 1.0; every request carries it, every answer carries the server's own, and HubProtocol.greet and HubProtocol.resolve refuse a server of another major with a sentence. HubServer is the side a client calls, one request object per exchange. The exchanges' schemas are HubProtocol.endpoints, snapshotted in api/flutter3d_hub.hub. No network code yet: the client is an addition on these types.