docs(C8): state output:vec![] as the sink declaration + eval return-width contract

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>

docs/design/INDEX.md

@@ -215,7 +215,15 @@ the record to a destination it holds as a field (a channel, a chart handle) —
 **out-of-graph side effect**. There is no `Sink` type, trait, or engine flag: a
 node that only records returns `None` (pure consumer), and a node may record
 **and** return an output the engine forwards in the same `eval` (the "both"
-case). In-graph routing stays engine-owned data (the edge table); the escape out
+case). **Encoding & return contract.** A pure consumer declares `output: vec![]`
+— the empty record *is* the sink declaration; there is no separate type, trait,
+or marker. Its `eval` returns `None` or a zero-width `Some(&[])`, and the run
+loop debug-asserts the returned row's width equals the declared output width
+(`row.len() == schema.output.len()`). 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` — making a sink
+structurally unwireable as an in-graph producer; its only output is the
+out-of-graph side effect. In-graph routing stays engine-owned data (the edge table); the escape out
 of the graph is the node's own side effect — and that boundary is the
 determinism / graph-as-data boundary (C1/C7).