feedback7: merge fb7-render — #382 赤刃明霄陈 S3 sword-qi drawn (sim chen3Wave fx), #381 enemy attack recovery held in interpolation, #380 push / pull slide (corpse stays where pushed; same-step kills finish the slide), #384 eye masks, 香槟炸弹 HP bar hidden, #246 an unwound attack plays its whole clip

- Conflicts with the merged fb7-hud (both added display-only snapshot lists): server/sim/battle/events.js, types/core.js,
  public/js/render/interp.js, docs/SIM.md, docs/design/network.md — union of ammo / wolves / neg and stand / standCut.
- Grok review: MERGE AFTER FIXES — the HIGH finding (same-step push kill) fixed on the branch (7a33bc7e).
- Golden: diy-073 / diy-074 (fx counts only; union OK); broad node --test 5442 (5441 pass, 0 fail, 1 skipped); lint 0
  errors; typecheck.

test/sim/displace-fx.test.js

@@ -0,0 +1,33 @@
+// The contract between server/sim/battle/displacement.js and render/app.js: a displacement puts a
+// ['fx', 'displace', x, y, { id }] tuple into the stream the client renders. The client must read the destination
+// from the tuple's own slots — e[4] carries the id only, and reading x/y from it silently produced NaN, which made
+// slideTo a no-op (player report: 有停顿但没有中间帧).
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { makeBattle, enemyRec } from '../helpers/battleHarness.js';
+
+const walker = (o = {}) => enemyRec({ key: 'enemy_walker', hp: 1e6, speed: 1, ...o });
+
+test('a displacement puts a `displace` fx into the event stream the client receives', () => {
+  const h = makeBattle({
+    defs: { enemies: { enemy_walker: walker({ speed: 0.01 }) } },
+    enemies: [{ key: 'enemy_walker', pos: [10, 6] }],
+    content: 'none', autoFinish: false,
+  });
+  h.step();
+  const e = h.enemy('enemy_walker');
+  h.b.drainEvents();                                   // clear the spawn/etc. batch
+  const moved = h.b.displace(e, { x: 1, y: 0 }, 2, { force: 1 });
+  assert.ok(moved > 1.8, 'the enemy moved: ' + moved);
+  const evs = h.b.drainEvents();
+  const fx = (evs || []).filter((t) => Array.isArray(t) && t[0] === 'fx');
+  const disp = fx.find((t) => t[1] === 'displace');
+  assert.ok(disp, 'the stream carries a displace fx: ' + JSON.stringify(fx));
+  // the fx rounds its coordinates to 2 decimals (events.js fx), the unit keeps full precision
+  assert.ok(Math.abs(disp[2] - e.x) <= 0.01, 'e[2] is the destination x: ' + disp[2] + ' vs ' + e.x);
+  assert.ok(Math.abs(disp[3] - e.y) <= 0.01, 'e[3] is the destination y: ' + disp[3] + ' vs ' + e.y);
+  assert.equal(disp[4] && disp[4].id, e.id, 'e[4].id is the displaced unit');
+  assert.equal(disp[4].x, undefined, 'e[4] carries no x — reading it gave NaN, so no slide ever ran');
+  assert.equal(disp[4].y, undefined, 'e[4] carries no y either');
+});
+