Spacevines, Comet Expulsion event balances (#21256)

Slightly increases potency of Spacevines random seeds, and increases
range of areas which can spawn them.

Makes Comet Expulsion event populate a whole bunch of meteors on the
overmap, but most of them are just traveling in random directions. Makes
it less trivial for bridge crew to just scoot one tile to the left and
everything's fine. Tested it multiple times on local instance to find
numbers that tended to make the overmap dangerous, but not bullet-hell
levels. It should be okay for the Horizon to get hit once or thrice-
dodging them all ought to be a really nice trick. Also increased minimum
spawn distance from the edges of the overmap on round start to prevent
comet spawn from deciding 'hey fuck you' and nuking you with 4 meteors
to the face and 0 time to maneuver. That's too mean, even for me.

changes:
- balance: "Spacevines event has more area types to potentially spawn
in."
  - balance: "Spacevines event potency_min increased from 70 -> 85."
- balance: "Comet Expulsion event chances slightly increased with
greater weighting towards Bridge Crew availability."
- balance: "Comet Expulsion event now generates many overmap hazards,
mostly in random directions, to make evasion less trivial."
- balance: "Comet Expulsion event meteors now have a small chance to do
greater damage."
- balance: "Horizon and other overmap visitable sites minimum spawn
distance from overmap edge increased from 2 to 4."
  - qol: "Provided additional admin logging for Comet Expulsion event."

code/modules/overmap/_defines.dm

@@ -1,6 +1,6 @@
 
 //How far from the edge of overmap zlevel could randomly placed objects spawn
-#define OVERMAP_EDGE 2
+#define OVERMAP_EDGE 4
 
 /// All sector overmap objects.
 /// Assoc list of stringified zlevel integer value (like `"1"` or `"42"` etc)