Add text engraving functionality to sliding lids

[Copilot]

This enhancement adds the ability to engrave custom text directly onto Gridfinity sliding lids, addressing the issue where labels can't be seen when the lid is closed.

New Features

Six new text parameters added to gridfinity_sliding_lid.scad:

• sliding_lid_text_enabled - Enable/disable text on lid (default: false)
• sliding_lid_text - Custom text content (default: "Gridfinity")
• sliding_lid_text_size - Font size in mm (0 = auto-size)
• sliding_lid_text_depth - Engraving depth in mm (default: 0.3)
• sliding_lid_text_font - Font selection (default: "Aldo")
• sliding_lid_text_position - Text alignment: left/center/right (default: "center")

Implementation

The text is rendered as engraved negative space on the lid surface using linear_extrude(). Key features include:

• Automatic sizing: When text_size = 0, font size is automatically calculated to fit lid dimensions
• Smart positioning: Text is properly centered vertically and aligned according to the position setting
• Non-breaking: Feature is disabled by default, preserving all existing functionality
• Compatible: Works with all existing sliding lid features (cutouts, lip styles, etc.)

Usage Example

sliding_lid_text_enabled = true;
sliding_lid_text = "SCREWS";
sliding_lid_text_size = 8;
sliding_lid_text_position = "center";

This allows users to identify bin contents even when sliding lids are closed, solving the visibility issue described in the original request.

Fixes #197.

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