Drop the chunk palette; chunks are plain u16 tile arrays

The 64-entry per-chunk palette was a wire-format artifact and capped a chunk
at 64 distinct tiles. A Chunk is now Box<[u16; 1024]> with flags derived from
the id on lookup — the same layout a binary on-disk format would use.

Map path becomes the first CLI argument; assets/map_large is a generated
256×256 stress map (loads in ~0.5 ms release). The snapshot test takes
SNAPSHOT_MAP.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

sim/src/lib.rs

@@ -9,7 +9,7 @@ mod sim;
 
 pub use sim::Sim;
 
-use chunk::{Chunk, TileDef, TileFlags};
+use chunk::Chunk;
 use map::TileMap;
 use world::World;
 
@@ -99,8 +99,8 @@ pub fn tile_collidable(tile_id: u16) -> bool {
 
 /// Build a world from a loaded tile map. The map's top-left tile sits at world (0, 0); the
 /// map is sliced into 32×32 chunks. Tiles inside a loaded chunk but outside the authored map
-/// become an invisible solid border (id 0, collidable), so the walkable world edge sits flush
-/// with the visible map rim instead of with the larger chunk boundary.
+/// get id 0 (solid, see `tile_collidable`), so the walkable world edge sits flush with the
+/// visible map rim instead of with the larger chunk boundary.
 pub fn load_world(map: &TileMap) -> World {
     let mut w = World::new();
     let max_cx = (map.width.saturating_sub(1) >> 5) as i16;
@@ -109,12 +109,8 @@ pub fn load_world(map: &TileMap) -> World {
         for cx in 0..=max_cx {
             let chunk = Chunk::generate(|lx, ly| {
                 let (wx, wy) = world::local_to_tile(cx, cy, lx, ly);
-                if wx < 0 || wy < 0 || wx >= map.width as i16 || wy >= map.height as i16 {
-                    return TileDef { tile_id: 0, flags: TileFlags(TileFlags::COLLIDABLE) };
-                }
-                let id = map.tile(wx, wy);
-                TileDef { tile_id: id, flags: map::tile_flags(id) }
-            }).expect("palette overflow during world gen");
+                map.tile(wx, wy) // 0 outside the map
+            });
             w.set_chunk(cx, cy, chunk);
         }
     }