Add tb/tb_ldpc_decoder.sv with Wishbone read/write tasks, version
register test, and all-zero codeword decode test. Add tb/Makefile
with lint and sim targets.
Fix two RTL bugs found during testbench bring-up:
- ldpc_decoder_core.sv: skip unconnected H_BASE columns (shift=-1)
in LAYER_READ, LAYER_WRITE, and SYNDROME states to prevent
out-of-bounds array access and belief corruption
- ldpc_decoder_core.sv: fix syndrome_ok timing race by adding
SYNDROME_DONE state so the registered result is available before
the early-termination decision
- wishbone_interface.sv: fix VERSION_ID typo (0xLD01 -> 0x1D01)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
