refactor(composites): extract Stage-1 composite-builders into their own crate

Dissolve the aura-engine -> aura-std dependency by moving the vol_stop /
risk_executor composite-builders out of aura-engine into a new aura-composites
crate. This restores the engine's domain-free layering, which the
composites-in-engine placement (cycle 0065) had inverted.

Why: the engine routes type-erased Scalar records and never names a concrete
node -- summarize_r reads the PositionManagement record positionally, by column
index, without linking aura-std (which is exactly why aura-std was a dev-only
dependency before 0065). Parking the node-naming composite-builders inside
aura-engine forced the engine's production code to reference the node library
for the first time (aura-engine -> aura-std), an upside-down layering: the
engine is the foundation the node library builds upon, not the reverse.

The clean home is a dedicated crate that depends on BOTH the engine's
GraphBuilder and the std nodes, leaving each lower crate untouched:

    aura-composites -> { aura-engine, aura-std } -> aura-core   (acyclic)
    aura-engine     -> aura-core only            (domain-free again)
    aura-std        -> aura-core only            (lean node library)

aura-std reverts to a [dev-dependency] of aura-engine (the engine's own E2E
tests -- stage1_r_e2e, random_sweep_e2e, ger40_breakout -- still drive real
std nodes through a bootstrapped Harness; that test-only edge creates no cycle).

Moved with history (git mv):
- crates/aura-engine/src/composites.rs           -> crates/aura-composites/src/lib.rs
- crates/aura-engine/tests/risk_executor.rs      -> crates/aura-composites/tests/
- crates/aura-engine/tests/vol_stop_composite.rs -> crates/aura-composites/tests/

Consumers rewired: aura-cli imports risk_executor / StopRule from aura_composites;
the moved tests import the builders from the crate under test. The intra-doc link
to RollMode and the module rationale doc are updated for the new home.

Behaviour-preserving: workspace build clean; clippy --all-targets -D warnings
clean; full suite 513 passed / 0 failed, identical to baseline (the two moved
test files run unchanged under aura-composites); cargo doc --no-deps clean (no
broken intra-doc links).

Cargo.lock

@@ -50,6 +50,7 @@ dependencies = [
 name = "aura-cli"
 version = "0.1.0"
 dependencies = [
+ "aura-composites",
  "aura-core",
  "aura-engine",
  "aura-ingest",
@@ -60,6 +61,16 @@ dependencies = [
  "serde_json",
 ]
 
+[[package]]
+name = "aura-composites"
+version = "0.1.0"
+dependencies = [
+ "aura-core",
+ "aura-engine",
+ "aura-std",
+ "serde_json",
+]
+
 [[package]]
 name = "aura-core"
 version = "0.1.0"