feat(phase1): implement RNG, SampleCBD, SampleNTT modules + xsim TBs

Phase 1 complete — all 4 leaf modules verified:
- rng_sync.v: 256-bit Galois LFSR PRNG (10/10 PASS)
- sample_cbd_sync.v: CBD sampler with keccak_core PRF (2560/2560 PASS)
- sample_ntt_sync.v: SHAKE-128 rejection sampling for A matrix (1536/1536 PASS)
- xsim Verilog TBs for sha3 module (tb_sha3_xsim.v, tb_sha3_xsim_simple.v, tb_keccak_core_xsim.v)

test_framework/modules/sample_ntt/test_plan.json

@@ -0,0 +1,27 @@
+{
+  "module": "sample_ntt",
+  "rtl_top": "sync_rtl/sample_ntt/sample_ntt_sync.v",
+  "rtl_deps": [
+    "sync_rtl/sha3/keccak_core.v",
+    "sync_rtl/sha3/keccak_round.v"
+  ],
+  "tb_cpp": "sync_rtl/sample_ntt/TB/tb_sample_ntt.cpp",
+  "simulator": "verilator",
+  "timeout_s": 300,
+  "cases": [
+    {
+      "id": "k2",
+      "description": "k=2: A[0][0] polynomial, compare with Python sampleNTT",
+      "params": {"k": 2},
+      "num_vectors": 3,
+      "tolerance": "bit_exact"
+    },
+    {
+      "id": "k4",
+      "description": "k=4: test multiple (i,j) pairs",
+      "params": {"k": 4},
+      "num_vectors": 3,
+      "tolerance": "bit_exact"
+    }
+  ]
+}