Object passable/opaque: editor checkboxes + doc/simplify cleanup

- ObjectDef gains passable/opaque fields (defaults: false/true); Board::is_passable
  consults the object before the grid cell, so objects block movement independently
  of the floor tile beneath them
- Editor Objects tab exposes Passable and Opaque checkboxes; changes survive save/load
- Add ObjectDef::new(x, y) to centralise placement defaults; remove triple
  default_glyph() call and unused Glyph import from main.rs
- Fix stale Behavior doc comment; update CLAUDE.md and ARCHITECTURE.md

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

kiln-core/src/game.rs

@@ -70,9 +70,8 @@ pub struct FontSpec {
 /// passability and opacity. Future properties (pushable, shootable, etc.) can
 /// be added here without changing call sites.
 ///
-/// For scripted objects (`Archetype::Object`), the behavior will eventually be
-/// determined by the Rhai script rather than this struct. See the Object variant
-/// for details. (TODO fix this comment)
+/// For scripted objects, passability and opacity are stored directly on
+/// [`ObjectDef`] and will eventually be overridable by Rhai scripts at runtime.
 #[derive(Copy, Clone, Debug)]
 pub struct Behavior {
     /// Whether an entity can walk through this cell.
@@ -226,6 +225,7 @@ pub struct ObjectDef {
 }
 
 impl ObjectDef {
+    /// Returns the default glyph for a newly placed object: tile 63 (`?`) in yellow on black.
     pub fn default_glyph() -> Glyph {
         Glyph {
             tile: 63,
@@ -233,6 +233,21 @@ impl ObjectDef {
             bg: Rgba8 { r: 0, g: 0, b: 0, a: 255 },
         }
     }
+
+    /// Creates a new object at `(x, y)` with default glyph and blocking behavior.
+    ///
+    /// Defaults: `passable = false`, `opaque = true`, no script. These match
+    /// the serde defaults in the map file format so new objects round-trip correctly.
+    pub fn new(x: usize, y: usize) -> Self {
+        Self {
+            x,
+            y,
+            glyph: Self::default_glyph(),
+            passable: false,
+            opaque: true,
+            script_name: None,
+        }
+    }
 }
 
 /// A portal that teleports the player to a named entry point on another board.