define manifest contract

[halfcyan]

Implements issue #98, defining the provider-neutral Odorobo VM manifest contract.

Included

• VmManifest with explicit desired and observed state
• CPU, memory, storage, network, placement, boot, cloud-init, and vsock fields
• Contract validation for versions, resources, storage sources, boot disks, networks, cloud-init, and vsock
• Explicit unknown-field rejection during deserialization
• Provider ownership documentation for storage and networking resolution
• Representative fixtures for minimal, storage-backed, networked, cloud-init, and vsock manifests
• Serialization, round-trip, validation, and generated-schema tests

This is the base PR for the stacked Cloud Hypervisor abstraction in PR #119.