fix: repeated keyboard input when INT 16h polls an empty 8042 buffer

[fusefib]

Description of Changes

Added a check in BiosKeyboardInt9Handler.Run() for the Intel 8042 output-buffer-full/data-pending status bit before reading keyboard data from port 0x60.

If no new keyboard data is pending, the handler acknowledges IRQ1 and returns without processing a scancode.

Rationale behind Changes

The in-memory INT 16h handler invokes INT 09h when the BIOS keyboard buffer is empty in order to fetch pending keyboard input.

However, BiosKeyboardInt9Handler.Run() previously read port 0x60 unconditionally. Intel8042Controller.ReadByte() intentionally returns the previous data byte when no new data is pending.

This meant that when INT 16h invoked INT 09h while the 8042 output buffer was empty, the previous keyboard scancode could be processed again as if it were new input. In affected software, this caused a single physical key press to be duplicated repeatedly.

The issue was reproduced with the text adventure game Solus (1988). With SOLUS.COM: pressing L once resulted in a rapid flood of l characters despite the input pipeline receiving exactly one key-down and one key-up event. Pressing Enter once similarly resulted in multiple Enter key events being processed.

Suggested Testing Steps

1. Run SOLUS.COM and wait for its text parser prompt.

2. Tap a letter key such as L once, or press Enter once.

   • Before this change, a single key press can be processed repeatedly.
   • After this change, a single key press should be processed exactly once.