Mörsergranate: impact marker on the ground until the explosion

The landing point is noted from the grenade's cEventFX 0x96 at launch; a
translucent red circle with the explosion radius (shared ImpactRadius) is
drawn after the world's ground, under the objects, and ends when that
grenade's impact gathers its targets. Ground -> screen projection in
game/engine/worldview.h.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/todo.md

@@ -42,15 +42,6 @@ Known so far: the periodic-hit aura (effect kind 0x12) uses
 mapped yet. Other arts' ranges are not mapped. Fits the data-driven arts
 item above (range as a property of an art, one place to compute it).
 
-Mörsergranate impact indicator (asked 2026-10-25, while reworking the dwarf's
-Kanonenschuss / Mörsergranate): show exactly where the grenade will land and
-its explosion radius, a red circle with low alpha on the ground. The landing
-point seems to be decided at launch (cCreature combat move code `0x5145d0`,
-case 0x43e: the target point, changed by a random offset when closer than
-200; passed as the position of the grenade's cEventFX 0x96), so the circle
-could be drawn from the launch until the explosion; to be confirmed with the
-diagnostic log. To be built after the radius-per-level change.
-
 ## Rename life leech to "omnivamp"
 
 Asked 2026-10-15: life leech (property 0x349, "Lebensentzug") becomes