Absolute paths in -G/parameter overrides break bit-reproducible simulation dumps

When regenerating a VCD corpus on a second machine to prove reproducibility, 8 of 9 dumps matched by sha256 but one differed only because its build passed a parameter like -Gmemfile="$ROOT/sw/prog.hex". The path string becomes a parameter value, so it is written into the dump (a string parameter's declared width changes with the path length, e.g. [303:0] vs [311:0]), even though the simulated behaviour is identical. Fix it by passing a fixed relative path (cd into a known dir first), or document that the checkout root must be the same string. Diff the VCD headers ($var lines) before assuming nondeterminism in the simulator.