[grytchat]

First piece of sharing several windows at once (GRYT-1905). Forwarding is already dynamic: every track in a room gets its own AddTrack on each subscriber. Ingest wasn't. Each peer got four receive-only transceivers at join (mic, camera, one screen video, one screen audio), and since the SFU makes every offer, a second window had no m-line to arrive on.

What changes

• renegotiate may carry {"screens": n}. An empty data, which today's clients send, behaves exactly as before.
• For n > 1 the SFU adds receive-only video and audio pairs until the peer has n, then re-offers as usual. It never removes them. An unused pair costs an idle m-line until the peer closes.
• n is capped at 8 (maxScreenShares). That isn't a product limit, it bounds what one client can make the SFU allocate. Each window costs the sender an encoder and upload long before it gets near 8.
• The added receivers go in a package-level sync.Map, and slotOf reports them as slotScreen or slotScreenAudio. So refusedBy gates them on share_screen like the first pair, and never on share_video. The entries are deleted when the peer is cleaned up.

Where to look

• slotOf in handler_join.go: an extra receiver is recognised by registry lookup, not by position. Forwarded tracks share the same peer connection, so index alone can't say what a later transceiver is for.
• ensureScreenSlots runs even while the peer isn't stable. The renegotiation is deferred as before, and the new transceivers ride along in the next offer.
• The registry is global state in the package. A per-handler map would mean threading it through refusedBy and its tests.

Tests: screenslots_test.go covers a pair per window, no growth on a repeat or smaller request, the ceiling, the second window's video and audio gated on share_screen, and cleanup. go test ./... passes.

Not in this PR: the client side. @gryt/voice still has one screen sender, and the server and client still track one share per person. Those come next, and nothing sends screens until they land.

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