feat: publish live audio graph (context + masterGain) on window.slopsmith.stems

[OmikronApex]

Lets the host shell observe/re-route the stem mix — the desktop renderer-bus feeder (feedBack#828) taps masterNode when the output device is exclusive-style, so multi-stem songs stay audible under WASAPI-exclusive output (feedBack Phase 2).

• New src/audio-graph-publish.js: publishAudioGraph() / retractAudioGraph().
• Published at both graph build sites (buffer path in main.js, streaming path in streaming.js); retracted in teardown, guarded by masterGain node identity since the AudioContext is reused across songs.
• Deployment-agnostic: the plugin only says "here is my master" — no sphere branching; a no-op in the Docker sphere.

Independent of the other Phase 2 PRs (can merge in any order). Tests: 47/47.

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• src/audio-graph-publish.js
• src/main.js
• src/streaming.js

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