Add thunder audio on lightning strike edges

- Play a soft thunder SFX (sfx_thunder_01..04 at volume 0.35) on the
  strobe's 0→>0 edge, once per strike; mid-strobe frames stay silent.
- Move `wasOn` latch after the thunder check so it reflects the previous
  frame's state (fixes arm-frame and edge detection).
- Add `thunder: false` config to silence the voice while keeping the bolt.
- Cover with tests: arm frame silent, one clap per strike, mute path.

dev/nebula.test.mjs

@@ -477,6 +477,52 @@ check('lightning: a strike frame draws the bolt (both stroked); a quiet frame cl
   return drewOnStrike && cleared(lScene.gfx[0]) && cleared(lScene.gfx[1]);
 })());
 
+// Thunder (the VOICE of the bolt) — a fresh instance + a sound-recording
+// fake scene (the Sfx seam: scene.sound.play + the audio-cache guard), so
+// the assertions are independent of the update history above.
+const tScene = makeFakeScene();
+const thunderPlays = [];
+tScene.sound = { play: (key, o) => thunderPlays.push({ key, volume: o?.volume }) };
+tScene.cache = { audio: { has: (key) => String(key).startsWith('sfx_thunder_') } };
+const THUNDER_KEYS = ['sfx_thunder_01', 'sfx_thunder_02', 'sfx_thunder_03', 'sfx_thunder_04'];
+const tL = new NebulaLightning(tScene, '#f0884a', '#e9f36d', lcfg, lFx);
+tL.create();
+const tEdge = (tL.phase % tL.interval) * 1000; // this cycle's strike boundary
+tL.update(tEdge, 16); // first frame: arms the detector (entered mid-strike → silent)
+check('thunder: the arm frame (a system entered mid-strike) stays silent', thunderPlays.length === 0);
+// A dark frame between strikes (the real cadence has a long one — the
+// window is 1.35s, the interval 8–15s) resets the 0→>0 edge.
+const DARK = (t) => tL.update(t, 16);
+DARK(tEdge + 1500); // just past the 1.35s strobe window → dark sky
+tL.update(tEdge + tL.interval * 1000, 16); // the NEXT strike: a real 0→>0 edge
+check('thunder: one clap on the strike edge — a far, soft thunder voice (0.35)', (() => {
+  return (
+    thunderPlays.length === 1 &&
+    THUNDER_KEYS.includes(thunderPlays[0].key) &&
+    Math.abs(thunderPlays[0].volume - 0.35) < 1e-9
+  );
+})());
+tL.update(tEdge + tL.interval * 1000 + 100, 16); // mid-strobe: the bolt is still up
+DARK(tEdge + tL.interval * 1000 + 1500); // …and then the sky darkens
+tL.update(tEdge + tL.interval * 2000, 16); // the strike AFTER next: a new edge
+tL.update(tEdge + tL.interval * 2000 + 100, 16); // …and its mid-strobe
+check('thunder: one clap per strike (mid-strobe frames stay silent)', thunderPlays.length === 2);
+
+// The `thunder: false` config silences the voice (the bolt still strikes).
+const qScene = makeFakeScene();
+const quietPlays = [];
+qScene.sound = { play: () => quietPlays.push(1) };
+qScene.cache = { audio: { has: () => true } };
+const qL = new NebulaLightning(qScene, '#f0884a', '#e9f36d', { ...lcfg, thunder: false }, lFx);
+qL.create();
+const qEdge = (qL.phase % qL.interval) * 1000;
+qL.update(qEdge, 16);
+qL.update(qEdge + qL.interval * 1000, 16);
+check('thunder: `thunder: false` silences the voice (the bolt still strikes)', (() => {
+  const drew = (g) => g.calls.filter((c) => c === 'line').length > 0;
+  return quietPlays.length === 0 && drew(qScene.gfx[0]) && drew(qScene.gfx[1]);
+})());
+
 check('lightning: camera-anchored (sky model); destroy is idempotent', (() => {
   lScene.cameras.main.scrollX = 2600;
   lScene.cameras.main.scrollY = 1500;

js/visuals/NebulaLightning.js

@@ -164,15 +164,15 @@ export class NebulaLightning {
       this.wasOn = false;
       return;
     }
-    this.wasOn = true;
-
     // THE STRIKE (data/sfx.json → thunder_01…04, soft): the bolt going
-    // off is the strobe's 0 → >0 edge (this.wasOn is reset on the dark
-    // frames above). The first frame only arms (a system entered
-    // mid-strike stays silent), and the throttle makes near-simultaneous
-    // frames one clap.
+    // off is the strobe's 0 → >0 edge — this.wasOn is still the
+    // PREVIOUS frame's state here (it was reset on the dark frames
+    // above, and latched AFTER this check). The first frame only arms
+    // (a system entered mid-strike stays silent), and the throttle
+    // makes near-simultaneous frames one clap.
     if (this.armed && !this.wasOn) this.thunderStrike(nowMs);
     this.armed = true;
+    this.wasOn = true;
 
     // THE BOLT: shape from (strike ordinal, system seed) — stable through
     // this strobe, different every strike, identical every run.