Joseph Doherty 2589774480 feat(mqtt): SparkplugCodec decode + golden payload vectors ...

Decodes Sparkplug-B wire bytes into a driver-side projection (SparkplugPayload /
SparkplugMetric / SparkplugValueKind) for the Task 21 ingest state machine.

- Never throws, for ANY input. It sits on MQTTnet's shared dispatcher thread, so
  an escaping exception would stall delivery for every subscription on the
  connection, not one tag. Garbage, truncation, zero-length, a valid protobuf of
  another schema and an over-nested Template all resolve to a verdict.
- Zero-length input is INVALID, not "a payload with no metrics" — protobuf would
  parse it as all-defaults, and that is how a truncated-to-nothing body gets
  mistaken for a well-formed one.
- Explicit presence throughout (Has{Seq,Name,Alias,Datatype,Timestamp}), never a
  zero-check: an NBIRTH legitimately carries seq = 0, and every DATA metric after
  a birth carries an alias with no name and no datatype.
- Values are projected RAW, boxed, with the value oneof reported as an explicit
  ValueKind — Absent / Null / Scalar / Unsupported all mean different things and
  three of them carry a null value. DataSet/Template/extension decode as
  Unsupported (v1 scope) rather than throwing or silently vanishing.
- ReinterpretSigned() undoes Sparkplug's two's-complement-in-an-unsigned-field
  encoding of Int8/16/32/64. Kept out of decode because a DATA metric carries no
  datatype — only the consumer, holding the birth's alias table, knows which
  applies. Skipping it publishes 4294967254 for a tag whose value is -42.
- Datatype is carried as the generated Org.Eclipse.Tahu.Protobuf.DataType; the
  SparkplugDataType map is Task 17's and is applied downstream (Task 21).

Golden vectors: nbirth.bin (seq=0, named/aliased catalog, a negative Int32, an
is_null metric, a DataSet metric) + ndata.bin (alias-only metrics, no name, no
datatype) are hand-built by SparkplugGoldenPayloads, committed, and pinned by a
drift guard that rebuilds and byte-compares them on every run — plus a test that
they actually reach the output directory, since a missing copy item is invisible
in source.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

2026-07-24 21:09:14 -04:00

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