Mythic Masters walk together: the room checks a keeper's stone and the arena by distance, not just region #3763

Found by the round 13 judge (#3722, PR #3761).

denNear in apps/mythic-masters/src/net/party.ts believes keeper:n:k, mythic:n and trail:n:k:j dens when the player's last pose is anywhere in region n. So a page anywhere in region 5 can claim mythic:5 and then send a "bound" report that plausible accepts, without walking to the arena or beating the keepers on the way.

The fix: the places are known: arenaOf in src/world/world.ts, stones in src/world/trail.ts, and the trail dens' spots from trailWilds. Add them to the table scripts/dens.ts writes (src/net/dens.json) and check the distance, the way wild cells get one (400 m, or what fits each kind).

Done when a stone or arena den told from far off in the right region is dropped, there is one row per kind in test/dens.test.ts, and the edge bundle stays near 20 KB gzipped. Pairs with the pose speed check, #3760.