Add LuaLaTeX font constructor example for Harfbuzz renderer

[Copilot]

Adds example demonstrating font manipulation via fonts.constructors.features.otf.register with Harfbuzz renderer, based on babel discussion #346.

Implementation

• examples/lualatex-font-constructor/font-constructor-example.tex: Working example showing custom font feature registration that modifies vowel width and color using virtual font commands
• examples/lualatex-font-constructor/README.md: Technical documentation covering manipulator functions, feature registration for base/node/plug renderers, and VF command usage
• examples/README.md: Examples directory index

Example

\directlua{
  function redden(f)
    local u = unicode.utf8
    for i, v in pairs(f.characters) do
      if i < 256 and u.find(u.char(i), '[aeiouó]') then
        v.width = tex.sp('7pt')
        v.commands = {
          { "special", "pdf: 1 0 0 rg" },
          { "char", i },
          { "special", "pdf: 0 g" }
        }
      end
    end
  end

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

\setmainfont[RawFeature=+redden, Renderer=Harfbuzz]{FreeSerif}

Demonstrates technique for language-specific typography, custom ligatures, and character-level font modifications since fonts.handlers.otf.addfeature doesn't support Harfbuzz.

Original prompt

latex3/babel#346

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