cleanup_old_buffers

Cleanup old buffers to free memory.

Keeps buffers for:

• Open files (is_open == true)
• Top 10 most recently used files (by last_viewed)
• The current_file being accessed (always keeps its buffer)

Sets file.buffer = null for all other files to free memory.

Process

1. Collect all files with buffers that are not open

   2. Sort by last_viewed (most recent first)

   3. Keep top 10, clear buffers for the rest

This is called automatically before creating new buffers to prevent unbounded memory growth.

Parameters:

current_file

The file currently being accessed (always keeps its buffer)