Hit tiles: positions round half to even (PRTS 作战机制); enemies held just inside the field edge

docs/SIM.md

@@ -541,7 +541,7 @@ returns their current `weight` (including modifiers). The same tick's count and
 list and budget (DESIGN §28.27); this is local to the kit, not a change to general displacement.
 
 **Hit areas (`body.js`, user playtest #5 item 10).** A regular enemy is a point: in a grid range when the tile of its
-position (`round(y)`, `round(x)`) is a range tile, in a radius when its position is (DESIGN §3). A huge enemy (巨型单位:
+position (`roundHalfEven(y)`, `roundHalfEven(x)` — PRTS 作战机制 "四舍六入五成双"; `Math.round` would take 2.5 to 3) is a range tile, in a radius when its position is (DESIGN §3). A huge enemy (巨型单位:
 `enemy.hitArea` from data/enemies.json `hitArea` — 假想敌：胄 / 管 / 盐风主教昆图斯 / 阿利斯泰尔 / “萨米的意志”, PRTS
 "受击判定区域为长4.95、宽2.95的长方形，向上偏移1.0") is hit anywhere on a rectangle `w` × `h` tiles centred on its position
 moved `dx` columns right / `dy` rows up: it occupies every tile the rectangle overlaps (`bodyKeys` — 5 × 3 tiles; PRTS