Mythic Masters walk together: the room checks how fast a pose moves #3760

Left over from #3700 (the den half is in the round 13 PR of #3722).

The room now checks a den's creature and level against the world seed, but it still takes a pose on the page's word. denNear and huntNear in apps/mythic-masters/src/net/party.ts measure from the last pose relayed for that player, so a page can send a pose beside any den in its region, then walk into it.

The fix: in src/edge/region.ts, keep the time of each seat's last pose on its attachment and hold a pose that moved further than a run plus a margin allows since then, within one region.

Watch for the jumps the game itself makes, which must still pass:

• landing beside a friend on Start (onLand in src/net/online.ts);
• being sent home after a loss (sendHome in src/world/world.ts);
• a new region (r changed), and a reconnect after the room slept.

Done when a pose that jumps across a region is held, those three jumps still go through, and there is one test for each.