Add diagnostic logging for dtr nil value issue

[FlovereX]

Description

Added diagnostic logging to help identify the source of the dtr nil value error occurring in dig_cuboid_impl.

Changes

• Added null check and logging in dig_cuboid() after calling setup_reboot() to verify the DTR instance is created
• Added parameter validation and logging in dig_cuboid_impl() to confirm the dtr parameter is received correctly
• These diagnostics will help pinpoint where the nil value originates when the error occurs again

Issue

The program was failing with: dig.lua:692: attempt to index local 'dtr' (a nil value)

This PR adds logging to help debug the root cause.