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_cbd/test_plan.json

@@ -0,0 +1,24 @@
+{
+  "module": "sample_cbd",
+  "rtl_top": "sync_rtl/sample_cbd/sample_cbd_sync.v",
+  "rtl_deps": ["sync_rtl/sha3/keccak_core.v", "sync_rtl/sha3/keccak_round.v"],
+  "tb_cpp": "sync_rtl/sample_cbd/TB/tb_sample_cbd.cpp",
+  "simulator": "verilator",
+  "timeout_s": 300,
+  "cases": [
+    {
+      "id": "eta2",
+      "description": "CBD with eta=2: random seeds, compare with Python reference",
+      "params": {"eta": 2},
+      "num_vectors": 5,
+      "tolerance": "bit_exact"
+    },
+    {
+      "id": "eta3",
+      "description": "CBD with eta=3: random seeds, compare with Python reference",
+      "params": {"eta": 3},
+      "num_vectors": 5,
+      "tolerance": "bit_exact"
+    }
+  ]
+}