docs(vrs): define host-local partition contract

[myobie]

Summary

• add a thin Host-local supervision requirements/spec pair for one (catalog, host) control-plane subject
• define catalog liveness separately from resident-supervisor liveness: any continuing canonical catalog agent keeps the catalog live across supervisor downtime; DING/sidecar-only survival does not
• require supervisor state to remain separately reportable, preserve the last-applied catalog, and forbid teardown inference from supervisor absence or incomplete/partitioned observation
• define the resolved catalog root and PTY root as stable mounted state paths while the catalog is live or its supervisor runs, while preserving ordinary catalog edit/sync semantics
• link the explicit coordinated relocation boundary to Define a guided migration operation for state-bearing st2 paths #85 and record that guided migration is not implemented
• retain the partition floor: last complete validated local desired state, acceptable temporary host skew, independent convergence, and factual/neutral peer reachability

This draft adds no vision, code, runtime behavior, global health classifier, cross-root discovery, database semantics, transport policy, deployment, remote attachment, or file-copy protocol.

Current-main integration

• merged live main 16097daac413ea0babd128f58f710b3d00b38488
• exact head 2c2cdb70337a8caaa3cff1bb7ba579af1f3b4774
• diff remains exactly the four VRS files, +179/-0
• requirement-number collision resolved as R18 LKG, live-main R19 targeted reconciliation, and R21 independent convergence; pending PR VRS: define runner-owned stable launch identity #80 retains R20
• current adoption, supervisor reporting, and DING-only evidence use immutable source links

Verification

• all local Markdown targets in the four touched files resolve
• all four new external evidence/issue URLs return HTTP 200
• cargo test --test invariants
• cargo test --doc --workspace
• cargo doc --workspace --no-deps (passes with five unchanged main rustdoc warnings)
• git diff --check origin/main...HEAD
• root docs/vrs/vision.md remains byte-unchanged

Hosted Nix succeeded on exact head 2c2cdb7: run 30545029988, job 90878900295.

Keep DRAFT with no reviewer or merge action pending Nathan hand review.