Add Cap'n Proto schema for CrioSphere and CrioZones

[LiGoldragon]

Motivation

• Provide a formal Cap’n Proto schema representing the inputs consumed by mkCrioSphere and the horizon outputs produced by mkCrioZones to enable serialization, tooling, and codegen.
• Capture the important domain types (nodes, users, machines, IO, trust, pre-criomes, and horizon metadata) in a single schema to reduce ad-hoc mappings and improve type safety.
• Make it easier to build integrations, validations, or RPCs that consume or emit the CrioSphere/CrioZones data structures.

Description

• Added capnp/criosphere.capnp which defines the schema for CrioSphereProposal and CrioZones along with supporting enums and structs such as Magnitude, NodeSpecies, UserSpecies, Machine, Io, NodeProposal, UserProposal, Horizon, HorizonNode, and HorizonUser.
• Introduced utility types including OptionalText, TrustEntry, BuilderConfig, NodeTypeFlags, SizedAtLeast, and other helper records that mirror the computed mkHorizonModule.nix structure (e.g. methods, behavesAs, computerIs).
• The schema mirrors the shape and semantics found in nix/mkCrioSphere and nix/mkCrioZones so generated data can round-trip between Nix evaluation outputs and typed consumers.

Testing

• No automated tests were executed because this is a schema-only change and does not affect runtime code paths.
• The new file was added and committed as capnp/criosphere.capnp in the repository.
• Consumers and codegen tooling have not yet been wired to this schema as part of this change.

---

Codex Task