Missing PAM write protection corrupts the VGA BIOS during Windows 98 shutdown

After installing [VMDisp9x](https://github.com/JHRobotics/vmdisp9x/releases/download/v1.2025.0.119/vmdisp9x-1.2025.0.119b-driver-2d.img) in Windows 98 SE, shutting down leaves a black screen with the CPU busy. Pressing a key makes CPU usage drop, but forcing a restart then runs ScanDisk because Windows did not shut down correctly.

During shutdown, the VGA BIOS clears `B8000–BFFFF`. This triggers Windows VDD's page-fault handling, which ends up overwriting `C0000–C7FFF`. `PAM1=0x11` makes that shadow RAM read-only, but [the code](https://github.com/copy/v86/blob/d96be774e549a83371b038b86e819804c96b921f/src/pci.js#L343) saves this value without protecting the memory. The VGA BIOS gets overwritten with `0x0720` text cells and shutdown never completes.

Honoring the PAM write-enable bits fixes it. [The slop86 fix](https://github.com/jlxip/slop86/commit/48bbf2e265ef07558a588c69d1adf2edba75ab41).

Start with src/pci.js around the linked line where PAM1 is saved, then compare the linked slop86 fix. Reproduce the Windows 98 SE shutdown failure with the VMDisp9x driver and verify that honoring the PAM write-enable bits prevents the VGA BIOS range from being overwritten and allows shutdown to complete.

Written by the indexing model from the issue text.