Runtime: make simultaneous first startup reliable #459

Simultaneous first startup can temporarily refuse; retry recovers. Runtime should complete first setup reliably while preserving the same authority checks.

Done means:

• Concurrent first starts complete consistently once setup succeeds.
• Interrupted startup and unavailable or invalid existing state retain clear recovery and the existing refusal rules.
• Process-level regression tests cover the ordering and recovery cases on supported platforms.
• Record exact-candidate checks, independent review and installed acceptance before closure.

Follow-up to #412. Coordinate store ownership with #203 and #443, and startup data-folder work with #357. Keep detailed diagnostics and source evidence in private review.

Acceptance record:

• Candidate: pending.
• Exercised checks: source and ownership review; implementation tests pending.
• Review links: pending.
• Installed release evidence: pending.
• Remaining gate: agreed design, implementation and regression proof.
• Next owner: store and startup owners, to be agreed.