diff --git a/bench/meson.build b/bench/meson.build
index b607ac65..1df36c15 100644
--- a/bench/meson.build
+++ b/bench/meson.build
@@ -115,7 +115,7 @@ bench_col_layout = executable(
   bench_thread_src,
   bench_workqueue_src,
   include_directories: [wirelog_inc, wirelog_src_inc],
-  dependencies: [nanoarrow_dep, threads_dep, xxhash_dep, mbedtls_dep],
+  dependencies: [nanoarrow_dep, threads_dep, xxhash_dep, mbedtls_dep, math_dep],
   install: false,
 )
 
diff --git a/docs/ARCHITECTURE.ko.md b/docs/ARCHITECTURE.ko.md
index ca507b8f..c50a48ae 100644
--- a/docs/ARCHITECTURE.ko.md
+++ b/docs/ARCHITECTURE.ko.md
@@ -110,7 +110,7 @@ Wirelog는 **순수 C11**로 구현된 **Timely-Differential 개념** 기반의
 - 워크큐 기반 병렬 실행에 자연스러운 적합성
 
 **코드 위치**:
-- 계획 수준: `wirelog/exec_plan.h:259` (WL_PLAN_OP_K_FUSION 연산자 타입)
+- 계획 수준: `wirelog/exec_plan.h` (WL_PLAN_OP_K_FUSION 연산자 타입)
 - 계획 생성: `wirelog/exec_plan_gen.c:1574-1746` (expand_multiway_k_fusion)
 - 실행: `wirelog/columnar/internal.h:1257` (col_op_k_fusion)
 - 워커 격리: `wirelog/columnar/diff_arrangement.h:88-98` (col_diff_arrangement_deep_copy)
diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index 6abc8648..ebf79a04 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -111,7 +111,7 @@ This document describes the **invariant architectural principles** that must be
 - Natural fit for workqueue-based parallel execution
 
 **Code locations**:
-- Plan-level: `wirelog/exec_plan.h:259` (WL_PLAN_OP_K_FUSION operator type)
+- Plan-level: `wirelog/exec_plan.h` (WL_PLAN_OP_K_FUSION operator type)
 - Plan generation: `wirelog/exec_plan_gen.c:1574-1746` (expand_multiway_k_fusion)
 - Execution: `wirelog/columnar/internal.h:1257` (col_op_k_fusion)
 - Worker isolation: `wirelog/columnar/diff_arrangement.h:88-98` (col_diff_arrangement_deep_copy)
diff --git a/tests/test_plan_exchange.c b/tests/test_plan_exchange.c
index cc322403..6a5675f5 100644
--- a/tests/test_plan_exchange.c
+++ b/tests/test_plan_exchange.c
@@ -393,6 +393,33 @@ test_plan_exchange_cleanup(void)
     PASS();
 }
 
+static void
+test_plan_op_abi_values(void)
+{
+    TEST("exec_plan op enum ABI values (Issue #495)");
+
+    /* Universal operators (0-8) */
+    ASSERT(WL_PLAN_OP_VARIABLE == 0, "VARIABLE should be 0");
+    ASSERT(WL_PLAN_OP_MAP == 1, "MAP should be 1");
+    ASSERT(WL_PLAN_OP_FILTER == 2, "FILTER should be 2");
+    ASSERT(WL_PLAN_OP_JOIN == 3, "JOIN should be 3");
+    ASSERT(WL_PLAN_OP_ANTIJOIN == 4, "ANTIJOIN should be 4");
+    ASSERT(WL_PLAN_OP_REDUCE == 5, "REDUCE should be 5");
+    ASSERT(WL_PLAN_OP_CONCAT == 6, "CONCAT should be 6");
+    ASSERT(WL_PLAN_OP_CONSOLIDATE == 7, "CONSOLIDATE should be 7");
+    ASSERT(WL_PLAN_OP_SEMIJOIN == 8, "SEMIJOIN should be 8");
+
+    /* Backend-specific boundary */
+    ASSERT(WL_PLAN_OP__BACKEND_START == 9, "__BACKEND_START should be 9");
+
+    /* Columnar backend operators (9-11) */
+    ASSERT(WL_PLAN_OP_K_FUSION == 9, "K_FUSION should be 9");
+    ASSERT(WL_PLAN_OP_LFTJ == 10, "LFTJ should be 10");
+    ASSERT(WL_PLAN_OP_EXCHANGE == 11, "EXCHANGE should be 11");
+
+    PASS();
+}
+
 /* ----------------------------------------------------------------
  * Main
  * ---------------------------------------------------------------- */
@@ -402,6 +429,7 @@ main(void)
 {
     printf("=== Plan Exchange Insertion Tests (#319) ===\n");
 
+    test_plan_op_abi_values();
     test_plan_tc_exchange_conservative();
     test_plan_no_exchange_nonrecursive();
     test_plan_exchange_key_metadata();
diff --git a/wirelog/columnar/eval.c b/wirelog/columnar/eval.c
index c238faac..6ad7456e 100644
--- a/wirelog/columnar/eval.c
+++ b/wirelog/columnar/eval.c
@@ -127,6 +127,17 @@ dedup_set_init_from_rel(col_rel_t *r)
 /* Stratum Evaluator                                                         */
 /* ======================================================================== */
 
+/* Compile-time check: WL_PLAN_OP__BACKEND_START must match the first
+ * backend-specific operator (Issue #495). */
+#if defined(_MSC_VER)
+/* MSVC C mode: use typedef array trick for compile-time check */
+typedef char static_check_backend_start_
+    [(WL_PLAN_OP_K_FUSION == WL_PLAN_OP__BACKEND_START) ? 1 : -1];
+#else
+_Static_assert(WL_PLAN_OP_K_FUSION == WL_PLAN_OP__BACKEND_START,
+    "WL_PLAN_OP__BACKEND_START must equal WL_PLAN_OP_K_FUSION");
+#endif
+
 /*
  * col_eval_relation_plan:
  * Evaluate all operators for one relation plan using the eval stack.
@@ -253,6 +264,7 @@ col_eval_relation_plan(const wl_plan_relation_t *rplan, eval_stack_t *stack,
             rc = col_op_exchange(op, stack, sess);
             break;
         default:
+            assert(0 && "unknown plan op type in columnar eval dispatch");
             break;
         }
         if (rc != 0)
diff --git a/wirelog/exec_plan.h b/wirelog/exec_plan.h
index cd0357de..312169c6 100644
--- a/wirelog/exec_plan.h
+++ b/wirelog/exec_plan.h
@@ -230,6 +230,15 @@ typedef enum {
  * Operator types in a backend execution plan.
  * Explicit integer values for stable ABI across backends.
  *
+ * Range partitioning (Issue #495):
+ *   Universal operators  (0-8):  Backend-agnostic relational algebra ops.
+ *                                Use flat fields in wl_plan_op_t only.
+ *   Backend-specific ops (9+):   Columnar backend optimizations.
+ *                                Use opaque_data for backend-defined metadata.
+ *                                See WL_PLAN_OP__BACKEND_START below.
+ *
+ * --- Universal operators (0-8) ---
+ *
  * WL_PLAN_OP_VARIABLE:    Reference to an input collection (EDB or IDB).
  * WL_PLAN_OP_MAP:         Column projection / rename.
  * WL_PLAN_OP_FILTER:      Predicate filter (expr in serialized buffer).
@@ -239,14 +248,20 @@ typedef enum {
  * WL_PLAN_OP_CONCAT:      Union of multiple collections.
  * WL_PLAN_OP_CONSOLIDATE: Deduplication / consolidation.
  * WL_PLAN_OP_SEMIJOIN:    Semijoin (SIP pre-filter).
+ *
+ * --- Columnar backend operators (9-11) ---
+ *
+ * WL_PLAN_OP_K_FUSION:    Parallel semi-naive delta expansion (Issue #370).
+ *                          opaque_data -> wl_plan_op_k_fusion_t.
  * WL_PLAN_OP_LFTJ:        Multi-way leapfrog triejoin on a single shared key
  *                          column across k >= 3 EDB relations (Issue #195).
+ *                          opaque_data -> wl_plan_op_lftj_t.
  * WL_PLAN_OP_EXCHANGE:    Redistribute tuples by hash(key_columns) % W across
  *                          workers for partition-correct parallel evaluation
- *                          (Issue #316).  opaque_data points to
- *                          wl_plan_op_exchange_t.
+ *                          (Issue #316).  opaque_data -> wl_plan_op_exchange_t.
  */
 typedef enum {
+    /* Universal operators (0-8): backend-agnostic */
     WL_PLAN_OP_VARIABLE = 0,
     WL_PLAN_OP_MAP = 1,
     WL_PLAN_OP_FILTER = 2,
@@ -256,11 +271,33 @@ typedef enum {
     WL_PLAN_OP_CONCAT = 6,
     WL_PLAN_OP_CONSOLIDATE = 7,
     WL_PLAN_OP_SEMIJOIN = 8,
+
+    /* Columnar backend operators (9+): use opaque_data */
     WL_PLAN_OP_K_FUSION = 9,
     WL_PLAN_OP_LFTJ = 10,
-    WL_PLAN_OP_EXCHANGE = 11, /* Redistribute tuples by hash(key) % W */
+    WL_PLAN_OP_EXCHANGE = 11,
 } wl_plan_op_type_t;
 
+/* First backend-specific operator value.  All ops with numeric value
+ * >= WL_PLAN_OP__BACKEND_START carry their metadata in opaque_data
+ * (defined by the active backend, e.g. columnar/columnar_nanoarrow.h).
+ * Universal ops (< WL_PLAN_OP__BACKEND_START) use flat wl_plan_op_t fields. */
+#define WL_PLAN_OP__BACKEND_START 9
+
+/** Returns true if @op is a backend-specific operator (>= BACKEND_START). */
+static inline bool
+wl_plan_op_is_backend_specific(wl_plan_op_type_t op)
+{
+    return (int)op >= WL_PLAN_OP__BACKEND_START;
+}
+
+/** Returns true if @op is a universal (backend-agnostic) operator. */
+static inline bool
+wl_plan_op_is_universal(wl_plan_op_type_t op)
+{
+    return (int)op < WL_PLAN_OP__BACKEND_START;
+}
+
 /* ======================================================================== */
 /* Operator Node                                                            */
 /* ======================================================================== */
@@ -274,6 +311,7 @@ typedef enum {
  *
  * Field usage by operator type:
  *
+ * Universal operators (use flat fields only, opaque_data is NULL):
  *   VARIABLE:    relation_name
  *   MAP:         project_indices, project_count  (and/or map_exprs)
  *   FILTER:      filter_expr
@@ -286,9 +324,11 @@ typedef enum {
  *   CONSOLIDATE: (no fields used)
  *   SEMIJOIN:    right_relation, right_filter_expr, left_keys, right_keys,
  *                key_count, project_indices, project_count
- *   K_FUSION:    opaque_data (points to wl_plan_op_k_fusion_t in columnar backend)
- *   LFTJ:        opaque_data (points to wl_plan_op_lftj_t in columnar backend)
- *   EXCHANGE:    opaque_data (points to wl_plan_op_exchange_t in columnar backend)
+ *
+ * Backend-specific operators (>= WL_PLAN_OP__BACKEND_START, use opaque_data):
+ *   K_FUSION:    opaque_data -> wl_plan_op_k_fusion_t (columnar)
+ *   LFTJ:        opaque_data -> wl_plan_op_lftj_t (columnar)
+ *   EXCHANGE:    opaque_data -> wl_plan_op_exchange_t (columnar)
  */
 typedef struct {
     wl_plan_op_type_t op;
@@ -315,11 +355,10 @@ typedef struct {
     wl_delta_mode_t delta_mode; /* semi-naive delta/full selection control */
     bool materialized; /* hint: cache this intermediate result for CSE reuse */
 
-    /* Backend-specific metadata.  NULL for all ops except K_FUSION.
-     * For K_FUSION: points to a wl_plan_op_k_fusion_t (defined in
-     * backend/columnar_nanoarrow.h) containing K operator sequences
-     * for parallel semi-naive evaluation.  Owned by the plan; freed
-     * via wl_plan_free() -> free_op() path. */
+    /* Backend-specific metadata.  NULL for all universal ops (< BACKEND_START).
+     * For backend-specific ops (K_FUSION, LFTJ, EXCHANGE): points to a
+     * backend-defined struct in columnar/columnar_nanoarrow.h.
+     * Owned by the plan; freed via wl_plan_free() -> free_op() path. */
     void *opaque_data;
 
     /* Filter predicate applied to right-child tuples before join probe.