Add total_parameter_size field to problem struct

Precompute total parameter size in problem_register_params, mirroring
how total_constraint_size is computed in new_problem.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Transurgeon

include/problem.h

@@ -62,6 +62,7 @@ typedef struct problem
     /* Parameter tracking for fast parameter updates */
     expr **param_nodes;     /* weak references to parameter nodes in tree */
     int n_param_nodes;
+    int total_parameter_size;
 
     /* Statistics for performance measurement */
     Diff_engine_stats stats;

src/problem.c

@@ -453,6 +453,10 @@ void problem_register_params(problem *prob, expr **param_nodes,
     prob->n_param_nodes = n_param_nodes;
     prob->param_nodes = (expr **)malloc(n_param_nodes * sizeof(expr *));
     memcpy(prob->param_nodes, param_nodes, n_param_nodes * sizeof(expr *));
+
+    prob->total_parameter_size = 0;
+    for (int i = 0; i < n_param_nodes; i++)
+        prob->total_parameter_size += param_nodes[i]->size;
 }
 
 void problem_update_params(problem *prob, const double *theta)