Mythic Masters walk together: a jump is believed only at a landing spot this player could reach #3791

Found by the round 15 judge (#3785, PR #3789).

landing() (apps/mythic-masters/src/net/dens.ts) believes a jump to any arena, campfire or trailhead in T.land, including ones in regions the player hasn't opened and campfires they never visited. So a page can teleport to any of them, be trusted at once, and walk to a den in a locked region.

The room has no save to check against, so a full fix needs some care. Options:

• A jump lands only in the player's current region, or a region next to it, unless it's their first pose.
• The page sends the region list it has opened. That is a claim, but it bounds how far a lie reaches.

Done when a test shows a jump to a locked region's campfire stays untrusted, honest travel still lands trusted (all 88 cases from round 14), and the price stays under $0.005 per player-hour.