Load maps from TOML files with XPM-style palette

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>

Cargo.toml

@@ -6,3 +6,5 @@ edition = "2024"
 [dependencies]
 eframe = "0.33.3"
 rhai = "1"
+serde = { version = "1", features = ["derive"] }
+toml = "0.8"