feedback1f WV: a knocked-out operator keeps its tile; 突袭 never lands on it

Player report F5 after 0.1.0: "突袭盟约的干员会移动到濒死干员的尸体上".

Real. A knocked-out operator left the occupancy map at once and only its
home tile stayed reserved. A 突袭 member that fell after its jump lay on the
landing tile, which nothing reserved, so the next member's jump (same target,
same scoring) landed on it; and a member's timed redeploy came back on its home
where a teammate that had jumped there lay. Real matches (six real 突袭
members, 战场#07 / #08, R6 / R9 / R12, 96 battles): 6 placements on a body
(3 jumps, 3 home redeploys) before, 0 of 526 jumps after.

Official (PRTS 卫戍协议/帮助, 作战阶段 / 单位部署):
- "干员退场后…原地留下一个“倒地干员”…满足再部署条件时，移除场上的该倒地干员
  并自动部署至该位置"
- "倒地干员所在地块视为可部署，但所有我方单位在此处的部署行为将被阻止"
- "若干员被击倒的位置为其他干员或召唤物的初始位置，则在被击倒后，尝试返回其
  自身的初始位置"

Now (sim/Battle.js):
- A knocked-out operator lies on `unit.body`: the tile it fell on, or its
  own home when it fell on another board piece's home and its home is free
  (one attempt at the knock-out, assumed). x / y / tileR / tileC keep where
  it fell, so the kill / death handlers still act there.
- Every redeploy comes back there (restTile): the timed one, 不屈's and
  阿戈尔's. A retreated operator still comes back on its home.
- downOn(r, c): _deploy (redeploys, the 突袭 landing, summons), spawnDevice
  and relocate refuse the tile for every ally of the field; isReservedTile
  reports it, so every automatic picker skips it (突袭 raidTile, tactical
  points, summon / device pickers, 维娜's lions). The kit helpers that kept
  their own home-tile loop (tier1 summonTileFree, tier3 / tier4 freeTile) go
  through isReservedTile.
- b.snap `down` entries gain the body tile: [id, respawnAt, respawnTime,
  state, row, col]. DOWN_STATE.WAIT_TILE becomes a safeguard only.

Tests: test/sim/feedback1f-downed-tile.test.js (real Match / PlayerState /
BattleSpec runs with real 突袭 chess, the 突袭 jump, the engine refusals,
the return home); downelem and bonds_addon follow the official rule.

server/sim/content/kits/tier4.js

@@ -119,10 +119,13 @@ const reveal = (battle, enemies) => { for (const e of enemies) if (e.s.flags.ste
 function shove(battle, e, from, force) {
   return e && e.alive ? battle.push(e, num(force, 0), { from }) : 0;
 }
-/** Free tile for a summon/device: in the rect, standable, empty and not the home tile of any ally (dead ones redeploy there). */
+/**
+ * Free tile for a summon/device: in the rect, standable, not reserved (Battle.isReservedTile: empty, no knocked-out
+ * operator lying there) and not the home tile of any ally (dead ones redeploy there).
+ */
 function freeTile(battle, r, c, { ranged = false, ground = false } = {}) {
   if (!Number.isInteger(r) || !Number.isInteger(c) || !battle.grid.inRect(r, c)) return false;
-  if (battle.unitAt(r, c)) return false;
+  if (battle.isReservedTile(r, c)) return false;
   if (!battle.grid.canStand(r, c, { ranged })) return false;
   if (ground && !battle.grid.groundPassable(r, c)) return false;
   for (const u of battle.allyUnits) if (!u.removed && u.homeR === r && u.homeC === c && u.kind !== 'device') return false;