[#1268] Single-writer across posture use + honest fallback (no-regression baseline) #1273

Acceptance Criteria

• Across a thin-client ⇄ Remote-SSH move, the canonical store has exactly one writer at every step (asserted).
• Remote-SSH unavailable/unconfigured → the client lands in the existing thin-client posture and the existing fleet-client relay suite passes unchanged (the named baseline).

Testing Decisions

Extend the never-fork + fleet-client relay suites: a cross-posture single-writer case and a fallback case asserting the named relay baseline stays green. Extends #1268-S1's surface.

Key Decisions

• Fallback reuses the existing thin-client posture verbatim — no new degraded path.

Constraints & Invariants

• Never-fork + one canonical writer; no silent fallback (the fallback is the honest existing posture).

Prior Art

• The never-fork suite; the fleet-client relay suite (the baseline); Remote-SSH as a first-class fleet posture (opt-in) — host-native tooling on the durable hub #1268-S1's adoption path.

Source

• Part of Remote-SSH as a first-class fleet posture (opt-in) — host-native tooling on the durable hub #1268 · ADR 0025 · Blocked by Remote-SSH as a first-class fleet posture (opt-in) — host-native tooling on the durable hub #1268-S1, Remote-SSH as a first-class fleet posture (opt-in) — host-native tooling on the durable hub #1268-S2.