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/dwarf-arts.md

@@ -46,3 +46,27 @@
 - The impact's gather call gets a radius of size × (1 + 0.1 × (level / 5)),
   with the owner's Mörsergranate level (`game::ArtLevel`). The growth 0.1 per
   step is our choice; not yet seen in the game (Borf's level is 2).
+
+## Mörsergranate impact marker (asked 2026-10-25, done 2026-10-26)
+
+"Show exactly where the grenade will land and its explosion radius, a red
+circle with low alpha on the ground."
+
+Measured:
+- The landing point is decided at launch (`0x5145d0` case 0x43e): the
+  target's position, or, closer than 200, a point 200 from the dwarf with a
+  +-15 jitter. It goes to the new grenade (type 0x3fa) in cEventFX 0x96,
+  constructed at `0x5169fa` (`cEventFX::Construct` 0x41e0a0, thiscall, ret
+  0x44: code, owner id, target id, position area / x / y / level, ...).
+- The impact gathers its targets at exactly that point, on several frames.
+  The grenade object stays about 30 s after the explosion.
+- Ground -> screen: `0x622700` (world units -> world pixels, the isometric
+  plane), then (pixel - camera +0x14/+0x18) / zoom +0xb60 + screen centre
+  (floating texts 0x41e546). Checked with the hero: the screen centre.
+- The world view draws the ground (`0x62d530`, ground quad list), then the
+  objects.
+
+Mod: the construct call notes grenade, owner, landing point and radius
+(`ImpactRadius`, shared with the hit check); after `0x62d530` a disc (alpha
+0x40) with a 2 px rim (alpha 0xa0) is drawn there, without depth test, until
+the impact of that owner's grenade at that point gathers its targets.