`+vcd` writes an FST file named `dump_file.vcd`, ignores the documented `=path` argument, and the documented default filename does not match the code

[badnikhil]

README.md:88 documents:

+vcd[=path/to/waveform.vcd] Generates a waveform of the simulation. By default, it will save it as dump.vcd.

Three parts of that do not match the behaviour on 2528e7d (v3.1.0). Verified with the Verilator
flow on Ubuntu 24.04.1, Verilator 5.020.

1. The =path argument is ignored. simulator/sim_top.sv:522-523:

if ($test$plusargs("vcd")) begin
    $dumpfile("dump_file.vcd");

$test$plusargs is a boolean presence test, so +vcd=/tmp/foo.vcd enables tracing and then
discards the path. This differs from the two sibling options documented immediately below it, which
both use $value$plusargs and do honour their argument:

• simulator/models/hdl/commit_log_behav.sv:21-23 —
  if (!$value$plusargs("commit_log=%s", logfile)) logfile = "signature.txt";
• simulator/models/hdl/konata_behav.sv:122-124 —
  if (!$value$plusargs("konata_dump=%s", dumpfile)) dumpfile = "konata.txt";

2. The default filename is dump_file.vcd, not dump.vcd. Hardcoded at sim_top.sv:523.

3. The file is FST, not VCD. simulator/verilator/verilator.mk:21 builds with --trace-fst,
so $dumpfile produces an FST container regardless of the .vcd extension:

$ ./sim +max-cycles=100000 +load=tb/tb_isa_tests/build/isa/rv64ui-p-jalr +vcd
$ file dump_file.vcd
dump_file.vcd: data
$ xxd -l 16 dump_file.vcd
00000000: 0000 0000 0000 0001 4900 0000 0000 0000  ........I.......
$ fst2vcd dump_file.vcd -o /dev/null   # exit 0

A .vcd extension on FST content breaks tooling that dispatches on extension. GTKWave itself
sniffs the container and opens it either way, so this is easy to miss.

Also unlisted: +start-vcd-cycles=N (sim_top.sv:524) defers $dumpvars() until cycle N,
and it is not in the README's optional-parameters section.

[badnikhil]

I am not sure about whats the best fix , but i think a doc update is just fine.