Memorial stones only for level 10 creatures, at most 30 per map

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -140,8 +140,8 @@ grid), guard and attack markers (shields and swords), savegames with the game st
    or trees with lightning every 6-20 s. Dimmed daylight, rain streaks, fog veils, flashes; a HUD label.
 6. **Named creatures** (done, v202611-00068, names.rs) - every creature gets a name from its kind's list in the
    language files (20 per kind, per language), a humorous epithet from level 5 and a grand one from level 8;
-   names in the hover info, the creature overview and the messages. A creature of level 3+ that falls leaves a
-   memorial stone on its master's ground (hover: who lies there; at most 24).
+   names in the hover info, the creature overview and the messages. A creature of level 10 that falls leaves a
+   memorial stone on its master's ground (hover: who lies there; at most 30 per map).
 7. **Hall of the fallen** (done, v202611-00073) - every named creature of level 3+ that falls is remembered
    with its level, heroes slain and how it fell (slain by a kind, lava, water, snare, lightning): in the game
    (creature overview "Fallen", or a click with the hand on a memorial stone) and over all games on the