feat(sdk): register human identities before sending

[noodleonthecape]

Summary

• add first-class human registration helpers that resolve to canonical routable identities
• auto-register human handles on send and surface a clear SDK-level registration error when registration fails
• preserve routable DM targets in the broker/message bridge instead of collapsing them to display names

Testing

• attempted package build in a clean worktree, but local toolchain deps were not installed there ( unavailable)
• Rust tests were not run locally here because cargo is unavailable in this environment

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