Brummel
f000373507
Closes the spec_gap from the cycle-0006 fieldtest
(docs/specs/fieldtest-0006-substrate.md): the public surface never stated that
an empty `output` is THE pure-consumer (sink) declaration, nor defined what a
`Some` return paired with an empty output means.
The behaviour was already principled and defended in the engine — this only
documents it (no code change):
- `output: vec![]` is the pure-consumer declaration; there is no separate
Sink type/trait/marker.
- `eval` must return a row whose width equals `schema.output.len()`, so a pure
consumer returns `None` or a zero-width `Some(&[])`; the run loop
debug-asserts the width (harness.rs).
- field-wise wiring resolves `Edge::from_field` against the producer's output
at bootstrap, so no edge can bind a field of a zero-output node (it fails
with `BadIndex`) — a sink is structurally unwireable as an in-graph
producer; its only output is the out-of-graph side effect.
Ledger C8 gains an "Encoding & return contract" clause; aura-core node.rs
rustdoc (module, NodeSchema, Node trait) updated from the pre-0006 "1..K,
producer or transformer" wording to the three-role (producer/consumer/both)
contract.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>