engine: node.register resets delivery readiness and silently drops deliver frames until agent.register/inventory.sync

[willwashburn]

Found while root-causing a cross-node "reply never injected" report (fixed on the relay side by AgentWorkforce/relay#1273 — that repro turned out to be drive-mode hold, but this engine hazard was confirmed during the trace and can produce the same symptom on its own).

Problem

The 6.0.3 broker cursor negotiation (b510538, #254) introduced an agent_scoped delivery-readiness gate with two silent failure modes:

• Every node.register resets the provider connection's ready-set to empty (src/engine/node.ts ~1499–1505; adapter src/adapters/node/realtime.ts ~362–375), and node-level replay is deliberately skipped for cursor-negotiating brokers (node.ts ~1527–1535). Identities only become ready again via a per-agent agent.register or inventory.sync.
• While an identity is not ready, live dispatch silently returns — routes/deliveryRouting.ts ~419–427 leaves the row queued with no log, no last_dispatch_error, no retry — and sendToProvider re-drops deliver frames at adapters/node/realtime.ts ~244–249 (only action.invoke frames are queued in memory; deliver frames are not).

If a broker re-sends node.register (reconnect, Wi-Fi blip, engine restart losing in-memory readiness) and does not follow with inventory.sync/agent.register for every hosted agent, every deliver frame to that node is dropped silently. Sending still works (token-authenticated REST), so the failure is asymmetric and invisible until the sender gets delivery.failed / ttl_expired ~1h later. nodeDeliver.ts ~132–140 gates ephemeral events on the same predicate, so all frames to the node go dark.

Suggested fixes

• Log (at warn) when a deliver frame is skipped for readiness, and stamp the delivery row (last_dispatch_error) so the drop is observable.
• Consider replaying/marking ready from the broker's inventory.sync-equivalent state automatically on re-register, or fail the node.register negotiation loudly when readiness can never be satisfied.
• Conformance tests cover the reset (__tests__/conformance/delivery.test.ts ~550–704, 921–999) but there's no test asserting the recovery path when a broker re-registers on the same connection without re-announcing agents.