feat(specs): add NXP-3 - Metadata Schema

[mfw78]

Summary

• Define storage-aware metadata schema for Nexum identities
• Protocol Buffers schema with Version Manifest as root
• Storage unit separation by update frequency (identity metadata, accounts, preferences, contacts)
• Account registry with native/provider discriminator using oneof for extensibility
• Identity metadata, preferences, and contact management structures
• Per-account signature audit trails for O(1) lookup/append operations

Context

NXP-3 defines the data structures for identity metadata. It's designed with storage efficiency in mind, separating data by update frequency so that frequently-changing data doesn't require re-uploading static data.

Key design decisions:

• Uses oneof with messages for account type discrimination (NativeAccount vs ProviderAccount)
• Provider accounts integrate provider_type and config directly in the message
• Swarm references used for images (64 bytes encrypted refs)
• No explicit size limits (implied by storage layer)

Closes nxm-rs/nexum#88
Related to nxm-rs/pm#3 (EPIC 0.0)

Test plan

• Verify Zola builds successfully
• Review Protocol Buffers definitions for correctness
• Confirm storage unit separation makes sense
• Verify oneof usage for account types