feedback5 O20: the 缚地 status (groundbind) — an enemy air unit counts as a ground unit and cannot move

ba.groundbind "目标变为地面单位，无法移动；使部分近地悬浮敌人掉落；对重量大于3的单位持续时间减半" (予愿安洁莉娜 S2 needs it):
- buffs.js: STATUS groundbind (flag groundbind + noMove, move x0), FLAG_KEYS, RESIST_STATUSES (as 束缚, [ASSUMED]);
- units.js isFlying: an enemy under 缚地 is a ground unit unless a 浮空 lifts it (same result as before without the flag);
- battle/status.js: 浮空 lands on a 缚地 data flyer (PRTS 异常效果 "…且不持有缚地异常…则Buff取消"), 缚地 halved on weight > 3;
- content/enemies: 掠海漂移体 / 吉兆飞鳞 drop on 缚地 as their PRTS text says (吉兆飞鳞 stays down while it holds);
- render/style.js: the 束缚 icon for it; docs/SIM.md (§2 isFlying, §3 flags / status table / 抵抗).
Nothing applies it yet: every golden family is unchanged. Test: test/sim/feedback5-groundbind.test.js.

server/sim/units.js

@@ -179,13 +179,18 @@ export class Unit {
   /**
    * Air unit (空中单位) for every targeting / ground-only rule: FLY movers, and enemies that hover (近地悬浮, buff flag
    * `float` — PRTS 术语释义 ba.float "算作空中单位"; they keep walking the ground path) or are levitated (浮空, gamedata_const
-   * ba.levitate "变为空中单位"). Movement and pathing read `motion`, never this.
+   * ba.levitate "变为空中单位"). A 缚地 enemy (status `groundbind`, ba.groundbind "目标变为地面单位") is a ground unit
+   * meanwhile — unless a 浮空 lifts it again (浮空 lands on a 缚地 flyer: Battle.applyStatus). Movement and pathing read
+   * `motion`, never this.
    */
   get isFlying() {
-    if (this.motion === 'FLY') return true;
-    if (this.side !== 'enemy') return false;
-    const f = this.s.flags;
-    return !!(f.float || f.levitate);
+    if (this.side === 'enemy') {
+      const f = this.s.flags;
+      if (f.levitate) return true;
+      if (f.groundbind) return false;
+      return this.motion === 'FLY' || !!f.float;
+    }
+    return this.motion === 'FLY';
   }
 
   get hpRatio() { const mh = this.s.maxHp; return mh > 0 ? this.hp / mh : 0; }