Introduces a map file format: TOML with a [palette] mapping characters
to (Glyph, Element) definitions and a [grid] multi-line string. Board
now owns the player position, objects, and portals. map_file::load()
deserializes a file and converts it to a Board via From<MapFile>.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
