#856 vlnggen's final link forgets the VCD object file

[ananthsforge]

Found by Claude Code

The last step of building the code model links a fixed list of object files. When Verilator is asked for a --trace build, it puts the waveform-dumping runtime, verilated_vcd_c.o, in the global object directory rather than inside the Vlng__ALL.a archive that vlnggen links. Vlnggen's object list is hardcoded and does not name it, so a trace-enabled build fails to link. Our script adds that object when it exists.