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-cli/tests/cli_run.rs

@@ -4336,7 +4336,7 @@ fn sweep_and_walkforward_refuse_trace_with_a_forward_pointer() {
 #[test]
 fn aura_sweep_persists_the_canonical_blueprint_keyed_by_topology_hash() {
     use aura_engine::{blueprint_from_json, blueprint_to_json};
-    use aura_std::std_vocabulary;
+    use aura_vocabulary::std_vocabulary;
     use sha2::{Digest, Sha256};
 
     let cwd = temp_cwd("blueprint-sweep-store");