test: add vector-driven Verilator testbench with Python model cross-check

Add gen_verilator_vectors.py to convert test_vectors.json into hex files
for $readmemh, and tb_ldpc_vectors.sv to drive 20 test vectors through
the RTL decoder and verify bit-exact matching against the Python model.

All 11 converged vectors pass with exact decoded word, convergence flag,
and zero syndrome weight. All 9 non-converged vectors match the Python
model's decoded word, iteration count, and syndrome weight exactly.

Three RTL bugs fixed in ldpc_decoder_core.sv during testing:
- Magnitude overflow: -32 (6'b100000) negation overflowed 5-bit field
  to 0; now clamped to max magnitude 31
- Converged flag persistence: moved clearing from IDLE to INIT so host
  can read results after decode completes
- msg_cn2vn zeroing: bypass stale array reads on first iteration
  (iter_cnt==0) to avoid Verilator scheduling issues with large 3D
  array initialization

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tb/vectors/expected.hex

@@ -0,0 +1,80 @@
+3FD74222
+00000001
+00000001
+00000000
+09A5626C
+00000001
+00000001
+00000000
+2FFC25FC
+00000001
+00000001
+00000000
+5DABF50B
+00000001
+00000001
+00000000
+05D8EA33
+00000001
+00000001
+00000000
+19AF1473
+00000001
+00000001
+00000000
+34D925D3
+00000001
+00000001
+00000000
+45C1E650
+00000001
+00000001
+00000000
+A4CA7D49
+00000001
+00000001
+00000000
+D849EB80
+00000001
+00000001
+00000000
+9BCA9A40
+00000001
+00000001
+00000000
+79FFC352
+00000000
+0000001E
+00000043
+5D2534DC
+00000000
+0000001E
+0000003B
+F21718ED
+00000000
+0000001E
+0000003D
+7FE0197C
+00000000
+0000001E
+00000041
+9E869CC2
+00000000
+0000001E
+0000004B
+4E7507D9
+00000000
+0000001E
+00000038
+BB5F2BF1
+00000000
+0000001E
+00000033
+AA500741
+00000000
+0000001E
+0000004C
+F98E6EFE
+00000000
+0000001E
+0000002A

tb/vectors/num_vectors.txt

@@ -0,0 +1 @@
+20