fix(ci): remove unused cexpr_node_by_id, guard csv memset for -Werror

macOS clang flags -Wunused-function on cexpr_node_by_id (defined
but never called). Ubuntu CI GCC -O3 flags -Wstringop-overflow on
memset with potentially negative ncols cast to size_t.

Author: singaraiona

src/io/csv.c

@@ -1279,7 +1279,7 @@ ray_t* ray_read_csv_opts(const char* path, char delimiter, bool header,
     /* ---- 8b. Pre-allocate nullmaps for all columns ---- */
     uint8_t* col_nullmaps[CSV_MAX_COLS];
     bool col_had_null[CSV_MAX_COLS];
-    memset(col_had_null, 0, (size_t)ncols * sizeof(bool));
+    if (ncols > 0) memset(col_had_null, 0, (size_t)ncols * sizeof(bool));
 
     for (int c = 0; c < ncols; c++) {
         ray_t* vec = col_vecs[c];

src/ops/query.c

@@ -315,9 +315,6 @@ static void cexpr_env_pop(ray_graph_t* g, int n) {
  * call (which may grow g->nodes via graph_alloc_node and invalidate
  * all previously-returned pointers).  The ID is stable; only the
  * backing address may change. */
-static inline ray_op_t* cexpr_node_by_id(ray_graph_t* g, uint32_t id) {
-    return &g->nodes[id];
-}
 
 /* Compile a Rayfall AST expression into a DAG node */
 static ray_op_t* compile_expr_dag(ray_graph_t* g, ray_t* expr) {