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>

crates/aura-core/src/node.rs

@@ -1,7 +1,8 @@
 //! The node contract (C8): the interface every node implements. A node declares
 //! its inputs (each with its scalar kind, lookback depth, and firing policy, C6)
-//! and its output record (1..K base columns, C7) via `schema`, and computes one
-//! cycle's row via `eval`.
+//! and its output record (0..K base columns, C7; an empty output declares a
+//! pure consumer / sink role, C8) via `schema`, and computes one cycle's row
+//! via `eval`.
 //! Tunable params (C12/C19) are deliberately not part of the schema yet — see
 //! spec 0002's "Out of scope".
 
@@ -42,18 +43,25 @@ pub struct FieldSpec {
 
 /// A node's declared interface: its inputs (in order) and its output record — an
 /// ordered list of named base columns; length 1 is a scalar (the degenerate
-/// case). Built once at wiring, never on the hot path — the `Vec`s are fine here.
+/// case), and an **empty** `output` (`vec![]`) declares a **pure consumer**
+/// (sink role, C8) — a node with no output port. Built once at wiring, never on
+/// the hot path — the `Vec`s are fine here.
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct NodeSchema {
     pub inputs: Vec<InputSpec>,
     pub output: Vec<FieldSpec>,
 }
 
-/// The universal composable dataflow unit (C8): one output port carrying a record
-/// of 1..K base columns, a producer or transformer. `schema` declares the
+/// The universal composable dataflow unit (C8): a **producer, consumer, or both**.
+/// A producer/transformer exposes one output port carrying a record of 1..K base
+/// columns; a **pure consumer (sink)** declares `output: vec![]` and records via
+/// an out-of-graph side effect in `eval` (cycle 0006). `schema` declares the
 /// interface; `eval` computes one cycle's row, returning a borrowed slice into a
-/// buffer the node owns (`None` = filter / not-yet-warmed-up). `&mut self` because
-/// a node may keep its own derived state (and its output buffer).
+/// buffer the node owns. `None` = filter / not-yet-warmed-up / pure sink; a
+/// returned `Some(row)` must satisfy `row.len() == schema().output.len()` (so a
+/// pure consumer returns `None` or an empty `Some(&[])`) — the engine debug-asserts
+/// the width and forwards the row field-wise along the node's out-edges. `&mut self`
+/// because a node may keep its own derived state (and its output buffer).
 pub trait Node {
     fn schema(&self) -> NodeSchema;
     fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Scalar]>;