docs: brokers are consumer nodes (C10), several attachable for comparable curves

Correct the broker mechanism: a broker is an ordinary downstream consumer node (C8/C9), not an external plugin/subsystem. It consumes the position-event stream + price streams and emits an equity stream; several brokers (e.g. sim-optimal pip + realistic currency) can be attached to the same position table at once, yielding directly comparable equity curves. Updates C10, CLAUDE.md invariant 7, aura-engine/aura-std crate docs, and the day-in-the-life doc.

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

crates/aura-std/src/lib.rs

@@ -2,8 +2,8 @@
 //!
 //! The universal, batteries-included building blocks every project gets for
 //! free: common indicators (SMA, ATR, RSI, …), resamplers, the `SessionNode`,
-//! standard combinators, and broker profiles. Depends only on `aura-core` (the
-//! shared contract).
+//! standard combinators, and broker nodes (the sim-optimal broker + realistic
+//! broker profiles). Depends only on `aura-core` (the shared contract).
 //!
 //! This is the top tier of the three-tier node-reuse model (contract C16):
 //!