docs: add Asahi Silverblue Nix workstation lane

[mdheller]

Summary

This PR adds the initial workstation-lane document for the current Apple Silicon preparation path:

• Asahi boot path
• Fedora Silverblue immutable base
• Nix tooling/runtime layer
• explicit shared-state staging

Why

We now have active upstream work split across:

• standards/protocol/bootstrap repos in SocioProphet
• runtime implementation in SourceOS-Linux/openclaw
• host/operator lanes in SociOS-Linux/workstation-contracts

The workstation side needs an upstream document that captures the install/staging/update/rollback posture before more runtime integration lands.

Scope

Docs only.
No machine-readable contract yet.

Follow-up

1. Turn this into a machine-checkable lane contract in this repo.
2. Add concrete proofs for install/update/rollback/shared-state checks.
3. Tie the runtime layer in once the lane contract is stable.