Mythic Masters: co-op PvE, two friends against a wild one (needs a 2v1 sim) #3929

The owner asked for three ways to fight with friends in Mythic Masters: PvP, co-op PvE, and both straight into the arena. The duel PR (fight a friend) shipped the PvP one: Qollab friends and challenges, then a lockstep duel on a Duel Durable Object (src/edge/duel.ts, src/net/duel.ts). This issue is the co-op one.

Why it wasn't in that PR: the fight sim is strictly one on one. Match.f is [Fighter, Fighter], every rule in src/sim.ts reads m.f[side ? 0 : 1] as "the other one", and sim-rs/src/sim.rs mirrors it bit for bit (CI checks every tick). Two players against one wild means a sim that knows teams.

What co-op needs:

1. Sim: fighters as a list with a team each. Targeting (the lock-on, a heavy's sweep, ranged shapes aimed at the target) picks the nearest foe. Body push between all pairs. KO ends the round when one team is down. Change sim.ts and sim.rs together; test/sim-rust.test.ts keeps them in step.
2. Bot: bot.ts picks a target (nearest, or whoever hit it last). opener.ts already wraps it.
3. Wire: Lockstep in src/net/duel.ts carries two inputs a tick. With N it's the same scheme: the room relays each seat's inputs to the others, and a tick plays when all are in. The Duel room grows to 2-4 seats, or a Coop room sits beside it.
4. Where: a wild den or a keeper while walking together (?room=) would be the natural door: a friend in the same region joins your fight. The other way in is "Fight a Friend → Team up vs the wilds".
5. HUD and camera: bars for 3+ fighters; the camera locks on your target.

Rough size: the sim and its Rust twin are most of it (a day or two with the tuner kept balanced); the room and page are smaller since the duel's lockstep carries over.