Follow Serve liveness for sandbox readiness

[SaladDay]

PR5c-2a: sandbox readiness follows Serve liveness, plus the protocol points the agent-host switch (2b) needs.

• Relay.Serving(ref) reports a live serve peer at the resource's generation. It is documented under "Run a relay".
• Lease renewal and each Environment's connection status follow a live sandbox_resources row whose resource is Serving. One Worker pass covers resources of every kind. It skips quiesced compute and is ordered against the quiesce publish.
• An Environment initialization that ends before any step took effect returns to pending and is retried. That covers a bind that fails typed, and a first step rejected resource_unavailable. The scanner claims only when the Environment's resource, if any, is Serving.
• A resume of an Environment the connection has not quiesced, as after a Runtime restart, succeeds and changes nothing.
• runtime-protocol.md states the supersede rule as the Runtime implements it.

Docs: runtime-protocol.md and sandbox-link-protocol.md, en and zh.

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