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>

docs/project-layout.md

@@ -73,8 +73,10 @@ recorded before it enters a backtest, and only with your per-session consent.
    **position table** (open/close events); the default **sim-optimal broker**
    projects it into a synthetic **pip**-equity, yielding a metrics table
    (trades, hit-rate, pip-P&L, max-DD, Sharpe) + a run record (manifest +
-   metrics) under `runs/`. Add `--broker pepperstone` to project the same table
-   through a realistic broker (currency, friction) for viability. (Contract C10.)
+   metrics) under `runs/`. Brokers are consumer **nodes**, and several can be
+   attached at once: add `--broker pepperstone` to get a realistic currency
+   curve *alongside* the default sim-optimal pip curve — two comparable equity
+   curves from the same position table. (Contract C10.)
 4. **Sweep + Monte-Carlo:** `aura sweep … --grid lookback=5..50,thresh=0.5..0.9
    --mc-seeds 1000` → a distribution of metrics, best params, robustness bands.
 5. **Compose:** "combine it with `momentum-filter` as a weighted sum" → Claude