refactor: split the aura-std roster into C28 layer crates

Phase 4 of the Stratification milestone. aura-std held four C28 ladder
layers in one roster; this cuts them into layer-aligned, aura-core-only
node crates so the import direction is enforced by the crate graph:

- aura-std        — engine nodes only (arithmetic/logic/rolling + sinks)
- aura-market     — session, resample
- aura-strategy   — bias, stops, sizer, cost-model machinery
- aura-backtest   — sim_broker, position_management
- aura-vocabulary — the relocated closed std_vocabulary roster

Node modules move verbatim (byte-identical renames); consumers are
rewired by import path only. A new structural test
(aura-vocabulary/tests/c28_layering.rs) asserts each node crate's
[dependencies] stay within its C28-permitted inner set, catching the
acyclic-but-outward violation the compiler misses.

Behaviour byte-identical: full workspace suite green (1448 tests), no
golden edited, clippy -D warnings clean. C28 Status block updated.

closes #288

crates/aura-engine/src/graph_model.rs

@@ -321,7 +321,8 @@ mod tests {
     use aura_core::{
         Cell, FieldSpec, Node, NodeSchema, PortSpec, PrimitiveBuilder, Scalar,
     };
-    use aura_std::{Bias, Recorder, Sma, Sub};
+    use aura_std::{Recorder, Sma, Sub};
+    use aura_strategy::Bias;
     use std::sync::mpsc;
 
     /// A bare node whose only purpose is to back a `PrimitiveBuilder` in a test.