fix(simulation): resolve clippy warnings in atlas modules

Replace HashMap/HashSet with BTreeMap/BTreeSet per D-030 determinism
rule. Fix while_let_loop, map_or simplification, collapsible if,
unsigned_abs casting, iterator indexing, and redundant wildcard arms.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/atlas/heightmap.rs

@@ -43,7 +43,7 @@ impl BodyHeightmap {
     /// Returns `true` if the cell at (row, col) is land (above sea level).
     #[inline]
     pub fn is_land(&self, row: u32, col: u32) -> bool {
-        self.get(row, col).map_or(false, |e| e >= self.sea_level)
+        self.get(row, col).is_some_and(|e| e >= self.sea_level)
     }
 }