feat(traps): activation VFX/SFX, explosion effect, and help section

Wire mimic/pit/arrow/laser media on activate, poison/explosion via effects, and document the scene editor and traps in Instructions.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/renderer/control/explosionSfx.ts

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+/** Звук и длительность эффекта «Взрыв» (`public/explosion.mp3` + WebM с альфой). */
+
+import { getEffectsSfxGain } from './effectsSfxGain';
+
+const EXPLOSION_SFX_VOLUME = 0.95;
+
+/** Запас, если метаданные не прочитались (длина webm ~3.97 с). */
+const DEFAULT_EXPLOSION_LIFE_MS = 4200;
+
+export function explosionEffectSoundUrl(): string {
+  return new URL('explosion.mp3', window.location.href).href;
+}
+
+/** VP8/VP9 WebM с `alpha_mode=1` — рендер через HTML `<video>`, не Pixi. */
+export function explosionEffectVideoUrl(): string {
+  return new URL('vfx/explosion/aerial-debris-smoke.webm', window.location.href).href;
+}
+
+let cachedExplosionSfxDurationMs: number | null = null;
+
+/** Длительность трека в мс (кэш после первого чтения метаданных). */
+export function getExplosionSfxDurationMs(): Promise<number> {
+  if (cachedExplosionSfxDurationMs !== null) {
+    return Promise.resolve(cachedExplosionSfxDurationMs);
+  }
+  const url = explosionEffectSoundUrl();
+  return new Promise((resolve) => {
+    const a = new Audio();
+    const done = (ms: number): void => {
+      cachedExplosionSfxDurationMs = ms;
+      a.removeAttribute('src');
+      resolve(ms);
+    };
+    a.addEventListener('loadedmetadata', () => {
+      const d = a.duration;
+      done(Number.isFinite(d) && d > 0 ? Math.round(d * 1000) : DEFAULT_EXPLOSION_LIFE_MS);
+    });
+    a.addEventListener('error', () => done(DEFAULT_EXPLOSION_LIFE_MS));
+    a.src = url;
+    a.load();
+  });
+}
+
+/** Длительность визуала ≈ max(звук, webm), с разумными пределами. */
+export async function getExplosionEffectLifeMs(): Promise<number> {
+  const sfxMs = await getExplosionSfxDurationMs();
+  const raw = Math.max(sfxMs, DEFAULT_EXPLOSION_LIFE_MS);
+  return Math.min(60_000, Math.max(600, raw));
+}
+
+export async function playExplosionEffectSound(lifeMs: number): Promise<void> {
+  try {
+    const rawMs = await getExplosionSfxDurationMs();
+    const target = Math.max(200, lifeMs);
+    const rate = Math.max(0.25, Math.min(4, rawMs / target));
+    const el = new Audio(explosionEffectSoundUrl());
+    el.volume = Math.max(0, Math.min(1, EXPLOSION_SFX_VOLUME * getEffectsSfxGain()));
+    el.playbackRate = rate;
+    void el.play().catch(() => undefined);
+  } catch {
+    /* ignore */
+  }
+}