feat(ntt): implement synchronous NTT core with Barrett modular reduction

Phase 2.1: Merged Path00+Path01 NTT engine.
- barrett_mul.v: Barrett modular multiplication (a·b mod 3329)
- butterfly_unit.v: Cooley-Tukey/Gentleman-Sande butterfly
- zeta_rom.v: 128-entry ROM with bit-reversed roots of unity
- ntt_core.v: 7-layer NTT FSM, 256×12-bit register file
- ntt_sync.v: valid/ready streaming wrapper

Verified: 13/13 vectors bit-exact vs Python NTT/NTTInverse

test_framework/modules/ntt/test_plan.json

@@ -0,0 +1,36 @@
+{
+  "module": "ntt",
+  "rtl_top": "sync_rtl/ntt/ntt_sync.v",
+  "rtl_deps": [
+    "sync_rtl/ntt/ntt_core.v",
+    "sync_rtl/ntt/butterfly_unit.v",
+    "sync_rtl/ntt/barrett_mul.v",
+    "sync_rtl/ntt/zeta_rom.v"
+  ],
+  "tb_cpp": "sync_rtl/ntt/TB/tb_ntt.cpp",
+  "simulator": "verilator",
+  "timeout_s": 300,
+  "cases": [
+    {
+      "id": "forward",
+      "description": "Forward NTT: 256-coeff polynomial -> NTT domain, vs Python NTT()",
+      "params": {"mode": "forward"},
+      "num_vectors": 5,
+      "tolerance": "bit_exact"
+    },
+    {
+      "id": "inverse",
+      "description": "Inverse NTT: 256-coeff NTT-domain -> normal domain, vs Python NTTInverse()",
+      "params": {"mode": "inverse"},
+      "num_vectors": 5,
+      "tolerance": "bit_exact"
+    },
+    {
+      "id": "roundtrip",
+      "description": "Round-trip: NTT(INTT(f)) == f",
+      "params": {"mode": "roundtrip"},
+      "num_vectors": 3,
+      "tolerance": "bit_exact"
+    }
+  ]
+}