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/src/project.rs

@@ -14,7 +14,7 @@ use aura_core::project::{
 };
 use aura_engine::ProjectProvenance;
 use aura_registry::{Registry, TraceStore};
-use aura_std::{std_vocabulary, std_vocabulary_types};
+use aura_vocabulary::{std_vocabulary, std_vocabulary_types};
 use sha2::{Digest, Sha256};
 use std::fmt;
 use std::path::{Path, PathBuf};