C-05 Ingest client

Responsibility: validate and batch-write; the single ingestion seam. Owns the failure mode of: ingest backpressure.

What it does

It subscribes to the observation namespace, validates each message against its schema, batches, and writes to the observation store. It is the only thing that writes observations. Nothing else has credentials to.

Why a single seam

Because "the only way in" is a property that can be tested, and "mostly the only way in" is not. Every guarantee about what the store contains — that the units are the declared units, that the observed property is one of the three the contract admits, that the timestamps came from the simulation clock — holds only if there is one place where those guarantees are applied.

It is also the only honest place to observe backpressure. When the broker delivers faster than the store accepts, something has to notice and say so. If writes are spread across several components, the pressure appears in several places at once and none of them has the whole picture.

What it deliberately is not

It is not an abstraction over intake with a pluggable interface. The requirements document is explicit that observation intake is aspirational as a port rather than real: there is one implementation and there is not a second one in prospect. Wrapping it in an interface would claim pluggability the system does not have, which is a rule drogna takes seriously enough to require a written argument before breaking it.

Requirements: FR-17, FR-18. Feature: 007.