When an unknown signal is encountered, total is set to -1 but the
loop continues. Subsequent known signals add their widths to -1,
producing incorrect totals (e.g. -1 + 16 = 15 instead of -1).
This can mask genuine truncation bugs in status word packing.
Three-tier test orchestrator validates opcode maps, bit widths, packet
layouts, and round-trip correctness across FPGA RTL, Python GUI, and
STM32 firmware. Catches 3 real bugs:
- status_words[0] 37-bit truncation in both USB interfaces
- Python radar_mode readback at wrong bit position (bit 21 vs 24)
- RadarSettings.cpp buffer overread (min check 74 vs required 82)
29 tests: 24 pass, 5 xfail (documenting confirmed bugs).
4th CI job added: cross-layer-tests (Python + iverilog + cc).