fix(aura-strategy): charge-basis-specific cost-node one-liners

The three cost nodes (ConstantCost, CarryCost, VolSlippageCost) shared a
byte-identical generic doc line, so `aura graph introspect --vocabulary`
could not tell them apart — C29's per-entry meaning was formally met but
not entry-specific (binary-only M1 fieldtest finding).

cost_node_builder now takes the factor-specific one-line doc as a
parameter (the shared string is gone from the single schema home); each
builder states its own charge basis. RED-first:
cost_node_descriptions_are_pairwise_distinct failed against the shared
string before the fix.

closes #330

crates/aura-strategy/src/carry_cost.rs

@@ -29,6 +29,7 @@ impl CarryCost {
             "CarryCost",
             Vec::new(),
             vec![ParamSpec { name: "carry_per_cycle".into(), kind: ScalarKind::F64 }],
+            "cost-model node: cost accrued per held cycle (param carry_per_cycle)",
             |p| Box::new(CarryCost::new(p[0].f64())),
         )
     }

crates/aura-strategy/src/constant_cost.rs

@@ -28,6 +28,7 @@ impl ConstantCost {
             "ConstantCost",
             Vec::new(),
             vec![ParamSpec { name: "cost_per_trade".into(), kind: ScalarKind::F64 }],
+            "cost-model node: fixed cost per trade (param cost_per_trade), charged at close",
             |p| Box::new(ConstantCost::new(p[0].f64())),
         )
     }

crates/aura-strategy/src/cost.rs

@@ -214,26 +214,20 @@ impl<F: CostNode> Node for CostRunner<F> {
 }
 
 /// Assemble a cost-node `PrimitiveBuilder`: the 4 geometry inputs ++ the factor's
-/// extra inputs, the standard 3-field cost output, the given params, and a build
-/// closure. The single home for the cost-node schema shape.
+/// extra inputs, the standard 3-field cost output, the given params, a
+/// factor-specific one-line `doc` (C29 — each cost node names its own charge
+/// basis rather than sharing one generic sentence, #330), and a build closure.
+/// The single home for the cost-node schema shape.
 pub fn cost_node_builder(
     name: &'static str,
     extra_inputs: Vec<PortSpec>,
     params: Vec<ParamSpec>,
+    doc: &'static str,
     build: impl Fn(&[Cell]) -> Box<dyn Node> + 'static,
 ) -> PrimitiveBuilder {
     let mut inputs = geometry_input_ports();
     inputs.extend(extra_inputs);
-    PrimitiveBuilder::new(
-        name,
-        NodeSchema {
-            inputs,
-            output: cost_output_fields(),
-            params,
-            doc: "cost-model node: charges its cost in R from position geometry, at close or accrued per held cycle",
-        },
-        build,
-    )
+    PrimitiveBuilder::new(name, NodeSchema { inputs, output: cost_output_fields(), params, doc }, build)
 }
 
 #[cfg(test)]
@@ -501,6 +495,7 @@ mod tests {
             "Probe",
             extra.clone(),
             params.clone(),
+            "cost-model node: probe factor for the builder-assembly test",
             |_p| -> Box<dyn Node> { Box::new(CostRunner::new(StubCost(0.0))) },
         );
         let schema = builder.schema();
@@ -537,6 +532,21 @@ mod tests {
         assert_eq!(a, "volatility");
     }
 
+    #[test]
+    fn cost_node_descriptions_are_pairwise_distinct() {
+        // C29: `aura graph introspect --vocabulary` must be able to tell the
+        // three cost nodes apart by their charge basis (flat-per-trade vs
+        // per-held-cycle-accrual vs volatility-scaled), not read one
+        // identical generic sentence three times (#330).
+        use crate::{CarryCost, ConstantCost, VolSlippageCost};
+        let constant_doc = ConstantCost::builder().schema().doc;
+        let carry_doc = CarryCost::builder().schema().doc;
+        let vol_slippage_doc = VolSlippageCost::builder().schema().doc;
+        assert_ne!(constant_doc, carry_doc, "ConstantCost vs CarryCost doc must differ");
+        assert_ne!(constant_doc, vol_slippage_doc, "ConstantCost vs VolSlippageCost doc must differ");
+        assert_ne!(carry_doc, vol_slippage_doc, "CarryCost vs VolSlippageCost doc must differ");
+    }
+
     #[test]
     fn producer_and_aggregator_share_the_triple() {
         // The structural lockstep: producer output and aggregator output/inputs all

crates/aura-strategy/src/vol_slippage_cost.rs

@@ -43,6 +43,7 @@ impl VolSlippageCost {
             "VolSlippageCost",
             volatility_input_ports(),
             vec![ParamSpec { name: "slip_vol_mult".into(), kind: ScalarKind::F64 }],
+            "cost-model node: slippage proportional to volatility (slip_vol_mult × volatility input), charged at close",
             |p| Box::new(VolSlippageCost::new(p[0].f64())),
         )
     }