feat(tb): add Vivado XSIM Verilog testbenches for all 10 sync modules

Add file-based vector testbenches ( + ) for:
- mod_add_sync, rng_sync, poly_arith_sync, comp_decomp_sync
- s_bram/sd_bram, sha3_chain_top
- ntt_core, poly_mul_sync
- sample_cbd_sync, sample_ntt_sync

Each module includes:
- tb_<module>_xsim.v: Vivado XSIM testbench
- gen_vectors.py: Python vector generator (stdlib only)
- vectors/<module>_input.hex: test input vectors
- xsim_run.tcl: compile + elaborate + simulate script

sync_rtl/sha3_chain/TB/xsim_run.tcl

@@ -0,0 +1,64 @@
+# xsim_run.tcl - Vivado xsim compilation and simulation script
+#
+# Compiles sha3_chain_top with all SHA3 dependencies plus testbench.
+# Run from the project root: ~/Dev/mlkem/
+#
+# Dependencies:
+#   sync_rtl/sha3/keccak_round.v    (combinational Keccak round)
+#   sync_rtl/sha3/keccak_core.v     (24-round Keccak core)
+#   sync_rtl/sha3/sha3_top.v        (SHA3 G/H/J top wrapper)
+#   sync_rtl/sha3_chain/sha3_chain_top.v  (ML-KEM G function)
+#
+# Prerequisites:
+#   source /opt/Xilinx/Vivado/2019.2/settings64.sh
+#
+# Usage:
+#   xsim -runall xsim_run.tcl
+#   vivado -mode batch -source xsim_run.tcl
+
+# ================================================================
+# Configuration
+# ================================================================
+set SHA3_DIR       sync_rtl/sha3
+set SHA3_CHAIN_DIR sync_rtl/sha3_chain
+set TB_DIR         sync_rtl/sha3_chain/TB
+
+# ================================================================
+# Step 1: Compile RTL sources (xvlog)
+# ================================================================
+puts "=== Compiling SHA3 RTL sources ==="
+
+# Core Keccak module (combinational round)
+xvlog -sv ${SHA3_DIR}/keccak_round.v
+
+# Keccak core (24-round sequential core)
+xvlog -sv ${SHA3_DIR}/keccak_core.v
+
+# SHA3 top wrapper (G/H/J modes)
+xvlog -sv ${SHA3_DIR}/sha3_top.v
+
+# sha3_chain_top (ML-KEM G function)
+xvlog -sv ${SHA3_CHAIN_DIR}/sha3_chain_top.v
+
+# ================================================================
+# Step 2: Compile testbench
+# ================================================================
+puts "=== Compiling testbench ==="
+
+xvlog -sv ${TB_DIR}/tb_sha3_chain_xsim.v
+
+# ================================================================
+# Step 3: Elaborate snapshot (xelab)
+# ================================================================
+puts "=== Elaborating snapshot ==="
+
+xelab tb_sha3_chain_xsim -s tb_sha3_chain_xsim
+
+# ================================================================
+# Step 4: Run simulation
+# ================================================================
+puts "=== Running sha3_chain test ==="
+xsim tb_sha3_chain_xsim -R
+
+puts ""
+puts "=== sha3_chain simulation complete ==="