Replace Element with Behavior + Archetype; update map format

Element is replaced by two types: Behavior (a plain struct of runtime
properties: passable, opaque) and Archetype (an enum of named presets:
Empty, Wall, Object, ErrorBlock). Board.cells changes from Vec<(Glyph,
usize)> to Vec<(Glyph, Archetype)>, eliminating the per-board element
palette. Map files now reference archetypes by name (archetype = "wall")
instead of property bags (passable = false), so the archetype list can
be reordered without breaking saved games. Unknown archetype names
produce an ErrorBlock cell (yellow ? on red) so malformed maps are
immediately visible in-game.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

maps/start.toml

@@ -5,8 +5,9 @@ height = 25
 player_start = [30, 12]
 
 [palette]
-" " = { passable = true,  ch = " ", fg = "#000000", bg = "#000000" }
-"#" = { passable = false, ch = "#", fg = "#808080", bg = "#606060" }
+" " = { archetype = "empty", ch = " ", fg = "#000000", bg = "#000000" }
+"#" = { archetype = "wall",  ch = "#", fg = "#808080", bg = "#606060" }
+"+" = { archetype = "banana",  ch = "#", fg = "#808080", bg = "#606060" }
 
 [grid]
 content = """
@@ -18,7 +19,7 @@ content = """
 #                                                          #
 #                                ######                    #
 #                                     #                    #
-#                                     #                    #
+#             +++                     #                    #
 #                                     #                    #
 #                                                          #
 #                                                          #