Implement fog substrate invariants

[mdheller]

Implement the merged fog substrate note as concrete SourceOS substrate artifacts.

Scope:

• formalize /srv/fog/* host contract
• define LVM/thin-pool conventions for fog-capable nodes
• define container-host baseline expectations
• document local CSI/LVM readiness assumptions

Acceptance:

• substrate-owned artifacts or implementation notes exist in-repo
• /srv/fog/projects, /srv/fog/models, /srv/fog/datasets, /srv/fog/topics, /srv/fog/vector, /srv/fog/cache, /srv/fog/logs, /srv/fog/secrets, /srv/fog/tmp are treated as first-class invariants
• boundaries remain intact: no topic semantics or settlement logic here