feat(mesh): let one node hold several rooms

[ForeverInLaw]

Why

A host that gives each conversation its own room has to start a node per conversation. Node identity is per process, so every one of those nodes presents the same peer id from a different port. A remote peer keeps one session per identity: it closes the rest on arrival, and connectPeer declines to dial the others at all because it already has that peer id connected.

Measured over three days, mosh clients only:

node	sessions	dead <1s	discarded handshakes/hour
A	15,494	14,799	205
B	9,772	9,419	131
C	8,527	8,112	113

95%+ of every session an active client opens dies inside a second, against only 6–8 distinct peers each. One identity appeared on 27 different ports inside a single hour. Those are full Noise handshakes — asymmetric crypto — thrown away.

What changed

A node is still born in one room, and every room-less call still means that room, so no existing behaviour moves. It can now join more:

• Moss_JoinRoom / Moss_LeaveRoom
• Moss_SubscribeRoom / Moss_UnsubscribeRoom / Moss_PublishRoom

The crypto is untouched — same derivation, same HMAC topic, same AEAD seal. What changed is which key is selected. That never needs guessing: a wire topic is an HMAC, so delivery reads the room off the subscription that created the topic instead of trying keys.

A room that was never joined is refused with MOSS_ERR_NOT_IN_ROOM rather than falling back to the node's own room — the fallback would publish where the intended peers are not listening and surface as lost messages hours later.

Compatibility

• Existing symbols unchanged; a host that never joins a second room sees nothing different.
• A joined room is byte-identical to the same room owned outright. TestJoinedRoomIsWireIdenticalToOwningIt pins exactly this: a consolidated client and an already-released one compute the same topic and can talk. Without it this change would silently cut off every shipped version.

Verification

• 4 new tests: wire-identity, isolation between rooms on one node, unjoined-room refusal, and delivery opening with the subscribed room's key.
• Each verified to fail when the key selection is reverted to single-room.
• Full suite green across all 14 packages; go vet ./... clean; c-shared build ok.