refactor(stage1-r): the vol stop is a composition, not a fused node

Applies the principle the user named: a node is a primitive only if it is NOT
DAG-expressible from other primitives; a function that needs a missing primitive
gets the primitive added, not fused away.

- Remove the fused VolStop node (it was pure feed-forward arithmetic).
- The volatility stop is now a composition of primitives, vol_stop(length, k):
  k * Sqrt(Ema(Mul(d,d), length)), d = Sub(price, Delay(price,1)) -- a rolling
  EWMA standard deviation. Built with GraphBuilder; proven end-to-end
  (bootstraps + runs + emits k*sigma) in tests/vol_stop_composite.rs.
- Migrate the one VolStop caller (stage1_r_e2e.rs) off it: the R-is-stop-defined
  test now contrasts a tight vs a wide FixedStop (two constant distances still
  fold to different R, the property under test).

Uses the Mul + Sqrt primitives added in the prior commit. FixedStop stays the
only stop-rule primitive (a triggered constant). Full suite + clippy green.

refs #117 #119

crates/aura-std/src/lib.rs

@@ -56,5 +56,5 @@ pub use session::Session;
 pub use sim_broker::SimBroker;
 pub use sma::Sma;
 pub use sqrt::Sqrt;
-pub use stop_rule::{FixedStop, VolStop};
+pub use stop_rule::FixedStop;
 pub use sub::Sub;