Update CLAUDE.md and ARCHITECTURE.md for Behavior/Archetype redesign and editor mode

Documents the replacement of Element with Behavior + Archetype, the
elimination of the per-board element palette, the new map file format
(archetype = "name" instead of passable = bool), the ErrorBlock sentinel,
and the editor mode (AppMode, EditorState, palette panel, click-to-paint).
Also updates "What's not yet implemented" to reflect editor progress and
call out the Object behavior / glyph chooser / file open gaps.

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

src/map_file.rs

@@ -172,4 +172,4 @@ pub fn load(path: &str) -> Result<Board, Box<dyn std::error::Error>> {
     let content = std::fs::read_to_string(path)?;
     let map_file: MapFile = toml::from_str(&content)?;
     Ok(Board::from(map_file))
-}
+}