feat(smash): spawn the Smash Crystal, so an ultimate is reachable without a command

[andrewgazelka]

MapSpec::crystals already parses "3 red data points for areas where the Smash
Crystal will spawn" out of every map file, and #997 added the half of the
mechanic that lives in the ability layer: kit::grant_ultimate hands a player
their kit's ultimate for ability::ULTIMATE_SECONDS, and ability::GrantedFor
counts that window down and takes the ability back when it lapses.

Nothing puts a crystal in the world. Until something does, the only way to reach
an ultimate is /crystal, which is what nix run .#smash-e2e uses to exercise
all fifteen of them.

What is missing is the pickup: an entity at one of the map's crystal points, a
respawn timer on it, and a collision that calls kit::grant_ultimate on whoever
walks into it. That is arena and terrain work rather than ability work, which is
why it was left.

Mineplex's version, for whoever picks this up: one crystal at a time, dropped at
a random one of the map's three points, respawning on a timer, and picking it up
gives the holder their kit's ultimate for a fixed window rather than a charge.

Suggested shape: an observer in terrain.rs or arena.rs that spawns the
entity on the transition into Phase::Playing, and a proximity check that calls
kit::grant_ultimate(&world, player, ability::ULTIMATE_SECONDS). Nothing under
module/ability.rs or module/kit.rs needs to change; the seam is already
there and /crystal can go once it exists.

Filed by an agent while wiring the ability registry in #997; the mechanic works
end to end today, only its trigger is missing.