filesys: reach motherboard and CPU-slot RAM through the GuestBus

[LinuxJedi]

Summary

Contributor-reported regression on big-box machines since the Ramsey motherboard RAM landed (#242/#244): an A3000/A4000 boot logged romtags: 0 ROM scsi.device resident(s) culled plus filesys: guest read/write outside RAM at 0x04xxxxxx.

Both symptoms are one gap. The services board's GuestBus -- the guest-memory view used for DosPacket handling and the scsi.device romtag cull, documented as "all RAM the CPU can see" -- only decoded chip, slow, and Zorro-board RAM. On an A3000/A4000 exec allocates its structures in the best RAM it has, which is now the motherboard bank ($04000000-$08000000 with 64M fitted), so:

• The cull walks ExecBase's ResModules list through the GuestBus; the list lives in motherboard RAM, every read returned 0, and the walk stopped at the first slot -- the ROM's scsi.device ran again, bringing back the several-second A4000 IDE probe (and the A3000 SCSI hang).
• The outside-RAM warnings were the same gap hit by packet traffic whose buffers sat in motherboard RAM.

The fix adds the Memory::mb_ram and Memory::accel_ram banks to the GuestBus decode. The zorro_device dma_* helpers stay 24-bit on purpose: they model real Zorro II bus-masters (A2091, CDTV DMAC), which cannot reach the 32-bit local banks.

Verification

• New regression test filesys::tests::guest_bus_reaches_motherboard_and_cpu_slot_ram runs the actual cull_rom_device through a GuestBus with the ResModules list in motherboard RAM (fails without the fix, culling 0), and round-trips an accelerator-bank access.
• End to end against the real KS 3.1 A4000 ROM with motherboard = "64M" and a [[filesys]] mount: the boot now logs romtags: 1 ROM scsi.device resident(s) culled, the host volume handler starts, and the outside-RAM warnings are gone.
• cargo test (1717 unit tests), cargo clippy, and cargo fmt --check are all clean.

[Copilot]

Pull request overview

Fixes a regression on big-box machines (A3000/A4000) where the filesys “GuestBus” guest-memory view did not decode Ramsey motherboard RAM and CPU-slot (accelerator) RAM, causing romtag culling (e.g., scsi.device) and DosPacket buffer accesses to fail when Exec allocated structures/buffers in those 32-bit local banks.

Changes:

• Extend GuestBus RAM decode to include Memory::mb_ram (at mb_ram_base()) and Memory::accel_ram (at ACCEL_RAM_BASE).
• Add a unit test that exercises romtags::cull_rom_device through GuestBus with the ResModules list placed in motherboard RAM, and validates read/write round-trips in accelerator RAM.

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