Capture received messages as per-message child effects in QueueManager

[stidsborg]

Summary

Iteration 1 of simplifying how messages get inlined into a flow. Each message a running flow receives is now durably captured as its own child effect the moment it is staged in ProcessMessages, and removed again when the message is delivered or idempotency-deduped — the same pattern IdempotencyKeys already uses, built on the new Effect.FlushlessCreateNextChild primitive (#186).

What changed

• ProcessMessages: on staging, creates a per-message child effect via FlushlessCreateNextChild(ReceivedMessagesRoot, EncodeMessage(message)), tracked in a new _pendingMessageChildren map (position → child id). Flushless → no I/O for the running flow.
• PruneDeliveredPendingMessage: on delivery/dedup, FlushlessClear(childId) deletes just that child.
• Initialize: rebuilds the map from GetChildren(ReceivedMessagesRoot) and re-stages children a prior incarnation left behind; a child whose position was already delivered is pruned rather than re-delivered (analogue of the existing delivered-positions store-row clear).
• PendingMessages.EncodeMessage/DecodeMessage made public for single-message round-trips.

Design decision

The children hang off a dedicated reserved id [-1,2] (ReceivedMessagesRoot), not PendingMessages.EffectId ([-1,1]). FlushlessClear deletes with clearChildren: true, so the completed-flow blob's own clear of [-1,1] would cascade-delete these children if they shared the parent. Separate ids keep the carriers independent — the completed-flow inline blob path (TryInlinePendingMessages, ExistingMessages) is untouched. A position landing in both is pruned by both branches (idempotent, self-correcting).

Sets up iteration 2

With the running flow capturing each message as a child, the completed-flow inline can move to the same representation next — TryInlinePendingMessages's merge-write-verify loop collapses into "append a child per message", and once both writers use children the [-1,1] blob and its bimodal prune branch go away.

Testing

• Core builds, 0 warnings.
• In-memory: 226 RFunctionTests + 100 messaging/effect/CreateNextChild — all pass.
• PostgreSQL: 95 messaging + the 11 completed-flow-inline & idempotency tests by name — all pass.