Wire BPHX channel pressure drop on both sides with selectable correlations.

Replace isobaric 4-port closures with SimplifiedChannel (default) and Martin1996 DP models so z_dp and UI dp_correlation actually affect the Newton solve.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/components/src/heat_exchanger/exchanger.rs

@@ -163,6 +163,15 @@ impl HxSideConditions {
     }
 }
 
+/// Live four-port edge indices: each triple is `(ṁ_idx, P_idx, h_idx)`.
+#[derive(Debug, Clone, Copy)]
+pub(crate) struct FourPortEdgeIndices {
+    pub hot_in: (usize, usize, usize),
+    pub hot_out: (usize, usize, usize),
+    pub cold_in: (usize, usize, usize),
+    pub cold_out: (usize, usize, usize),
+}
+
 /// Generic heat exchanger component with 4 ports.
 ///
 /// Uses the Strategy Pattern for heat transfer calculations via the
@@ -516,7 +525,7 @@ impl<Model: HeatTransferModel + 'static> HeatExchanger<Model> {
     }
 
     /// `true` when all 4 edges are wired (Modelica-style 4-port mode).
-    fn edges_ready(&self) -> bool {
+    pub(crate) fn edges_ready(&self) -> bool {
         self.hot_in_idx.is_some()
             && self.hot_out_idx.is_some()
             && self.cold_in_idx.is_some()
@@ -525,6 +534,60 @@ impl<Model: HeatTransferModel + 'static> HeatExchanger<Model> {
             && !self.cold_fluid_id_str.is_empty()
     }
 
+    /// Live four-port edge index triples `(ṁ, P, h)` when wired.
+    pub(crate) fn four_port_edges(&self) -> Option<FourPortEdgeIndices> {
+        if !self.edges_ready() {
+            return None;
+        }
+        Some(FourPortEdgeIndices {
+            hot_in: self.hot_in_idx?,
+            hot_out: self.hot_out_idx?,
+            cold_in: self.cold_in_idx?,
+            cold_out: self.cold_out_idx?,
+        })
+    }
+
+    /// Thermal-model equation count (excludes the two pressure-closure rows).
+    pub(crate) fn model_n_equations(&self) -> usize {
+        self.model.n_equations()
+    }
+
+    /// Calibration state indices (e.g. free `z_dp` actuator).
+    pub(crate) fn calib_indices_ref(&self) -> &entropyk_core::CalibIndices {
+        &self.calib_indices
+    }
+
+    /// Hot-side fluid id used for live property queries.
+    pub(crate) fn hot_fluid_id_str(&self) -> &str {
+        &self.hot_fluid_id_str
+    }
+
+    /// Cold-side fluid id used for live property queries.
+    pub(crate) fn cold_fluid_id_str(&self) -> &str {
+        &self.cold_fluid_id_str
+    }
+
+    /// Inlet density [kg/m³] from the attached fluid backend at `(P, h)`.
+    pub(crate) fn side_density(
+        &self,
+        side: &str,
+        fluid_id: &str,
+        p_pa: f64,
+        h_jkg: f64,
+    ) -> Result<f64, ComponentError> {
+        self.query_live_property(side, fluid_id, Property::Density, p_pa, h_jkg)
+            .and_then(|rho| {
+                if rho.is_finite() && rho > 1e-10 {
+                    Ok(rho)
+                } else {
+                    Err(ComponentError::DomainViolation(DomainViolation {
+                        component: Some(self.name.clone()),
+                        detail: format!("{} {}-side density is invalid: {}", self.name, side, rho),
+                    }))
+                }
+            })
+    }
+
     fn live_state_required_error(&self) -> ComponentError {
         ComponentError::InvalidState(format!(
             "{} requires live four-port edge state (hot_inlet, hot_outlet, cold_inlet, cold_outlet); inlet-only boundary conditions cannot define outlet states",