engine: first boxes of the round-trips plan (LTM drift gate, salsa::Update derives)

src/simlin-engine/src/common.rs

@@ -702,7 +702,7 @@ pub enum ErrorKind {
     Variable,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash, salsa::Update)]
 pub struct Error {
     pub kind: ErrorKind,
     pub code: ErrorCode,

src/simlin-engine/src/db.rs

@@ -42,7 +42,11 @@ mod dep_graph;
 mod element_graph_proptest;
 mod invariance;
 pub(crate) use invariance::model_flows_invariant;
-mod ltm_ir;
+// `pub(crate)` (not private-to-`db`) so the Track-A classifier-agreement gate,
+// mounted under `crate::ltm_augment`, can reach the production Expr2 walker
+// (`collect_all_reference_sites`) and the reference-site IR entry
+// (`model_ltm_reference_sites`) it compares the Expr0 partial builder against.
+pub(crate) mod ltm_ir;
 mod macro_registry;
 mod units;
 mod var_fragment;

src/simlin-engine/src/db/ltm_ir.rs

@@ -317,7 +317,7 @@ struct WalkCtx<'a> {
 /// left-to-right DFS over the AST, matching `enumerate_agg_nodes`, so the
 /// per-source site `Vec`s are deterministic (a salsa requirement on the
 /// cached IR result).
-fn collect_all_reference_sites(
+pub(crate) fn collect_all_reference_sites(
     target_var: &crate::variable::Variable,
     variables: &HashMap<Ident<Canonical>, crate::variable::Variable>,
     dim_ctx: &crate::dimensions::DimensionsContext,

src/simlin-engine/src/ltm_augment.rs

@@ -5908,3 +5908,14 @@ fn generate_nonlinear_partial(
 #[cfg(test)]
 #[path = "ltm_augment_tests.rs"]
 mod tests;
+
+// The Track-A differential gate: the Expr2 and Expr0 access-shape classifier
+// families must agree per non-reducer reference occurrence, so the
+// ceteris-paribus live reference survives PREVIOUS-wrapping instead of
+// silently zeroing the link score. Mounted here (not in `ltm_ir`) so it sees
+// this module's private Expr0 classifier (`classify_expr0_subscript_shape`)
+// and `IteratedDimCtx`; it reaches the Expr2 side through `pub(crate)`
+// `db::ltm_ir`.
+#[cfg(test)]
+#[path = "ltm_classifier_agreement_tests.rs"]
+mod classifier_agreement_tests;