Pass the FDS BIOS through HotSwap so disk images can be hot swapped

[UndocumentedWorlds]

Problem

HotSwap() calls Init(null, rom, null). On the FDS code path, Init()
throws before the board is created:

// NES.cs:382
if (fdsbios == null)
    throw new MissingFirmwareException("Missing FDS Bios");

So hot swapping to any .fds image fails immediately, regardless of
firmware configuration. Cartridge swaps are unaffected because Init()
never reads the third argument on the iNES/UNIF path.

Change

The constructor already resolves the BIOS. This keeps that array and
passes it through on swap.

Using the stored array rather than re-fetching it matters: the
constructor applies the 40976-byte bad-dump correction, and re-fetching
would skip it.

Notes

• Internal RAM preservation is untouched — the existing hotSwapping
  flag still does that work.
• The FDS board is recreated by Init(), so the 32 KB PRG-RAM is not
  carried across. It is reloaded from the new disk anyway.
• The header test mirrors the one inside Init(), so both headered
  .fds files and raw disk images (\x01*NI) are handled. Passing the
  array unconditionally would behave the same; the check is there to
  keep the intent readable.

Testing

• FDS → FDS swap mid-run: the new disk boots and $0000-$07FF is
  preserved across the swap.
• Normal loads of FDS and cartridge ROMs still work.

Check if completed:

• I have run any relevant test suites
• I, the committer, have read the licensing terms for contributors (last updated 2022-07-15) and am compliant