feat(poly_arith): implement synchronous PolyAdd/PolySub streaming module

Phase 2.3: Polynomial modular addition and subtraction.
- poly_arith_sync.v: mode=0 add (a+b mod Q), mode=1 sub (a-b mod Q)
- Pure streaming (1 coeff/cycle, no BRAM needed)
- Uses pipeline_reg for valid/ready handshake

Verified: 10/10 vectors bit-exact vs Python reference

test_framework/modules/poly_arith/test_plan.json

@@ -0,0 +1,24 @@
+{
+  "module": "poly_arith",
+  "rtl_top": "sync_rtl/poly_arith/poly_arith_sync.v",
+  "rtl_deps": ["sync_rtl/common/pipeline_reg.v"],
+  "tb_cpp": "sync_rtl/poly_arith/TB/tb_poly_arith.cpp",
+  "simulator": "verilator",
+  "timeout_s": 30,
+  "cases": [
+    {
+      "id": "add",
+      "description": "Polynomial modular addition: (A[i] + B[i]) mod Q for 256 coeffs, 5 random vectors",
+      "params": {"mode": "add"},
+      "num_vectors": 5,
+      "tolerance": "bit_exact"
+    },
+    {
+      "id": "sub",
+      "description": "Polynomial modular subtraction: (A[i] - B[i]) mod Q for 256 coeffs, 5 random vectors",
+      "params": {"mode": "sub"},
+      "num_vectors": 5,
+      "tolerance": "bit_exact"
+    }
+  ]
+}