Function render_glyph 

pub fn render_glyph(
    parser: &TtfParser<'_>,
    ch: char,
    pixel_size: u16,
) -> Result<GlyphBitmap, FontError>

Render a character to a grayscale bitmap at the given pixel size.

This is the main entry point for glyph rendering. It:

1. Looks up the glyph ID from the character code via cmap.
2. Parses the glyph outline from glyf.
3. Rasterizes the outline to a bitmap.