feat(components): add ThermoState generators and Eurovent backend demo

crates/solver/src/jacobian.rs

@@ -67,6 +67,26 @@ impl JacobianMatrix {
         JacobianMatrix(matrix)
     }
 
+    /// Updates an existing Jacobian matrix from sparse entries in-place.
+    ///
+    /// The matrix is first zeroed out, then filled with the new entries.
+    /// This avoids re-allocating memory during iterations, satisfying the
+    /// zero-allocation architecture constraint.
+    ///
+    /// # Arguments
+    ///
+    /// * `entries` - Slice of `(row, col, value)` tuples
+    pub fn update_from_builder(&mut self, entries: &[(usize, usize, f64)]) {
+        self.0.fill(0.0);
+        let n_rows = self.0.nrows();
+        let n_cols = self.0.ncols();
+        for &(row, col, value) in entries {
+            if row < n_rows && col < n_cols {
+                self.0[(row, col)] += value;
+            }
+        }
+    }
+
     /// Creates a zero Jacobian matrix with the given dimensions.
     pub fn zeros(n_rows: usize, n_cols: usize) -> Self {
         JacobianMatrix(DMatrix::zeros(n_rows, n_cols))