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>

src/main.rs

@@ -110,9 +110,9 @@ impl eframe::App for App {
             );
         });
 
-        // Force a repaint every frame so input is processed continuously.
-        // Remove this if the game becomes purely event-driven.
-        ctx.request_repaint();
+        // egui repaints automatically on input events, so no explicit
+        // request_repaint() is needed while the game is purely event-driven.
+        // Add it back (or call it from game logic) when continuous animation is needed.
     }
 }