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>
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