fix: enhance colour palette support

[MungaSoftwiz]

PR: Terminal Screen Rendering with Enhanced Colour Palette Support
close #29

Description
This PR improves the screen rendering system by introducing robust handling of three colour modes (Monochrome, 16 Colours, 256 Colours) and better-organised palette management. The system now supports:

• Accurate RGB-based palette definitions.
• Dynamic generation of the 256-colour palette, including the 6x6x6 colour cube and grayscale ramp.
• Improved error handling and more reusable rendering logic.

Key Changes

Palette Definitions:

• Monochrome: Two colours ({r:0, g:0, b:0} and {r:255, g:255, b:255}).
• 16 Colours: Includes the base ANSI palette (Black, Blue, Green, etc.).
• 256 Colours: Dynamically generated:

1. First 16 indices mirror the 16-colour palette.
2. Indices 16–231 represent a 6x6x6 RGB colour cube.
3. Indices 232–255 form a grayscale ramp.

Screen Setup:

Validation of screen dimensions and colour modes.
Palette initialisation based on the supplied mode.

Error Handling:

Added exceptions for invalid inputs, e.g., unsupported colour modes or out-of-range indices.

Documentation:

• Clear comments explaining palette definitions and rendering logic.
• Examples of how indices map to colours.

Known Issues/Limitations

• Non-ANSI terminals may not render the colours accurately.
• Customisation of the palette is not currently supported but could be considered for future enhancements.

Screenshot:

• Shows a 256 Colours mode which is now supported.
  

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