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/graph_construct.rs

@@ -2,7 +2,7 @@
 //! JSON op-list document deserializes into the `OpDoc` DTO (so the engine `Op`
 //! stays serde-free), which maps 1:1 into `aura_engine::Op`. The `aura graph
 //! build` / `aura graph introspect` subcommands here drive these ops through the
-//! injected `aura_std::std_vocabulary`.
+//! injected `aura_vocabulary::std_vocabulary`.
 
 use std::collections::BTreeMap;
 use std::path::Path;
@@ -16,7 +16,7 @@ use serde::Deserialize;
 // `std_vocabulary` is now only reached through `crate::project::Env::resolve` in
 // production code; tests still exercise it directly.
 #[cfg(test)]
-use aura_std::std_vocabulary;
+use aura_vocabulary::std_vocabulary;
 
 /// The wire DTO for one construction op — the document's by-identifier shape,
 /// internally tagged by `"op"`, mapped into the serde-free engine `Op`. Bind