Lane C S13: sky.setMute(on) — the tap A's M key was waiting on

One flag plus a gain write, but the flag is the point: a mute pressed on the
splash screen, before the first gesture builds the audio graph, is remembered
and honoured by unlock(). main.js already feature-detects sky.setMute, so the
M key lights itself up on this landing. audio.masterGain getter added so the
assert reads the real bus, not the flag back.

Selftest 336/0/0 (was 335, +1 test). Mutation-checked both ways: a no-op
setMute and an unlock that forgets a pre-unlock mute both go red.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/world/js/skyfx.js

@@ -342,8 +342,11 @@ function cloudTexture(size = 256, seed = 7) {
 // ---------------------------------------------------------------- audio
 // Synthesized layers. WebAudio won't start until a gesture (browser rule), so
 // everything is built lazily on unlock() and silently absent before it.
+const MASTER_GAIN = 0.55;
+
 function createAudio(seed = 1) {
   let ctx = null, master = null;
+  let muted = false;
   let windGain, windFilter, windHowl, howlGain;
   let rainGain, rainFilter;
   let gustGain, gustFilter;
@@ -380,6 +383,15 @@ function createAudio(seed = 1) {
     /** 'running' | 'suspended' | 'closed' | 'none'. `ready` only means the graph
      *  got built — a suspended context is still silent, so the HUD reports this. */
     get state() { return ctx ? ctx.state : 'none'; },
+    /** A's M key (gate 3.3). A flag, not just a gain write, so muting works
+     *  before unlock() has built the graph and survives it. */
+    setMute(on) {
+      muted = !!on;
+      if (master) master.gain.value = muted ? 0 : MASTER_GAIN;
+    },
+    get muted() { return muted; },
+    /** Real bus gain, for asserts — null until unlock() builds the graph. */
+    get masterGain() { return master ? master.gain.value : null; },
     /** Current layer gains — for the HUD and for asserting the bed tracks wind. */
     levels() {
       if (!started) return null;
@@ -399,7 +411,9 @@ function createAudio(seed = 1) {
       ctx = new AC();
       if (ctx.state === 'suspended') ctx.resume();
       master = ctx.createGain();
-      master.gain.value = 0.55;
+      // honour a mute that landed BEFORE the first gesture built the graph —
+      // M on the splash screen must not be forgotten by the unlock
+      master.gain.value = muted ? 0 : MASTER_GAIN;
       master.connect(ctx.destination);
       noiseBuf = noiseBuffer(ctx);
 
@@ -792,6 +806,16 @@ export function createSkyFx(o = {}) {
     /** Wire to the first click/keydown — browsers won't start audio otherwise. */
     unlockAudio() { audio.unlock(); },
 
+    /**
+     * A's M key (gate 3.3): mute the whole audio bus. main.js feature-detects
+     * this (`typeof sky.setMute === 'function'`) and only advertises M once it
+     * exists — so this method appearing is what lights the key up. Safe at any
+     * time: before unlock it sets a flag the unlock honours, after unlock it
+     * writes the master gain directly.
+     */
+    setMute(on) { audio.setMute(on); },
+    get muted() { return audio.muted; },
+
     /**
      * @param {number} dt
      * @param {number} t   storm time

web/world/js/tests/c.test.js

@@ -172,6 +172,27 @@ export default async function run(t) {
       `skyfx left ${scene.children.length - before.children} object(s) in the scene`);
   });
 
+  // SPRINT13 gate 3.3 — A's M key. main.js feature-detects sky.setMute and only
+  // advertises M once it exists, so this contract is what lights the key up.
+  // The pre-unlock case is the one that bites: M pressed on the splash screen,
+  // BEFORE the first gesture builds the audio graph, must survive the unlock.
+  t.test('M-mute: setMute silences the bus, and a pre-unlock mute survives unlock', () => {
+    const wind = createWind(storms.storm_02_wildnight);
+    const sky = createSkyFx({ wind });               // headless — the bus needs no scene
+    assert(typeof sky.setMute === 'function', "no setMute — A's M key has nothing to call");
+    assert(sky.muted === false, 'a fresh sky must not start muted');
+    sky.setMute(true);                               // before unlock — the splash case
+    assert(sky.muted === true, 'setMute(true) did not take');
+    sky.unlockAudio();
+    if (sky.audio.ready) {                           // no AudioContext = nothing to silence
+      assert(sky.audio.masterGain === 0,
+        `unlock forgot a pre-unlock mute: master at ${sky.audio.masterGain}`);
+      sky.setMute(false);
+      assert(sky.audio.masterGain > 0, 'unmute left the master gain at 0');
+    }
+    sky.dispose();
+  });
+
   // --- SPRINT2 §Lane C.3: rain has to stop at the cloth ---
   // Driven with a synthetic 4×4 m panel rather than a whole cloth sim: the thing
   // under test is the projection, and a flat panel makes the right answer