Remove the OutOfOrderSource constraint from composeSignal. The agent's
selection order is authoritative for source ordinals; timestamps remain
as per-source provenance metadata. This resolves the contract conflict
where reversed message selections (valid agent compositions) were
rejected by the primitive.
Each source still retains its original createdAt, so chronological
provenance is preserved exactly. The sourceKey includes ordered message
IDs, so different orderings produce distinct signals with deterministic
idempotency.
Coverage added:
- Primitive: non-chronological order accepted, timestamps preserved
- Backend: reversed selection preserves ordinals and exact raw text
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