docs: fix three drifts found in the session audit

C8: a node is producer/consumer/both — at most one output, pure consumers (sinks: chart/equity/logger) have none; records the node-role taxonomy the substrate requires (was wrongly 'exactly one output', contradicting pure consumers and C14). C3: merge by timestamp, normalizing source-native units (data-server Unix-ms) to the canonical epoch-ns of C7. aura-engine lib doc: broker is a downstream plugin over the position table (sim-optimal pip / realistic currency), not an in-strategy 'broker/portfolio node'. Plus a provenance line-wrap nit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/aura-engine/src/lib.rs

@@ -3,7 +3,9 @@
 //! Home of (to be specified): the ingestion boundary (k-way merge of timestamped
 //! sources into one chronological cycle stream), the deterministic synchronous
 //! single-threaded sim loop (one unique state per input tick), freshness-gated
-//! recompute, the broker/portfolio node mechanics, and the atomic sim unit
+//! recompute, the strategy's broker-independent position-event output and the
+//! downstream broker plugins that project it into equity (sim-optimal pip /
+//! realistic currency), and the atomic sim unit
 //! `(topology + params + data-window + seed) -> metrics` that the sweep /
 //! optimize / walk-forward / Monte-Carlo axes orchestrate.
 //!