Add LuaLaTeX font constructor example with documentation

[Copilot]

Adds reference implementation demonstrating font manipulation in LuaLaTeX using font constructors with Harfbuzz renderer, based on babel discussions #346.

Structure

• examples/ - New directory for cross-domain reference implementations
• examples/lualatex-font-constructor/ - Self-contained example with documentation
  • README.md - Technical documentation covering font constructor API, VF commands, and use cases
  • font-constructor-example.tex - Working demonstration modifying vowel properties (width, color)

Implementation

The example registers a custom font feature that modifies character properties at load time:

fonts.constructors.features.otf.register {
  name = "redden",
  manipulators = {
    base = redden,
    node = redden,
    plug = redden  -- Harfbuzz renderer
  }
}

Useful for language-specific typography, custom ligatures, and character-level visual effects where fonts.handlers.otf.addfeature doesn't support Harfbuzz.

Original prompt

Reference: e6007ef

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