Dwarf caves in all modes: neutral dwarves guard a hoard and sometimes a rune chest; the rune forge tower; imps repair towers before claiming

- 1-4 cave slots by map size, 50% each (at least one); knight treasure caves only where a mission places them
- dwarves fight intruders near the cave, never chase far, heal at home, grow with distance and the player's army, never count for victory
- rune chest: one free rune forge tower (chain bolt to three more enemies), upgradeable for gold, sold once it comes back (next mission / free play at once), unbuilt ones carry over
- new tool (key N) shown in the tower menu while a rune tower can be built or upgraded

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

lang/en.lang

@@ -625,3 +625,17 @@ hud.rival1 = Green
 hud.rival2 = Blue
 msg.creature_freed = A {0} breaks its chains and joins you!
 msg.spring_taken = A mana spring is yours: more mana and a higher maximum.
+tool.rune.label = Rune
+tool.rune.name = Rune Forge Tower
+tool.rune.desc = Plans from a dwarven chest: built for free, one tower per chest. Huge range, rune bolts jump on to three more enemies. Sell it and the base tower comes back once, in the next mission.
+tower.rune = Rune Forge Tower
+cost.rune = free: {0} left (upgrades {1} / {2} gold)
+hint.rune_none = No rune forge plans left. Claim a dwarven chest for a new one.
+msg.rune_chest = A dwarven chest! The plans of a rune forge tower are yours - build it for free from the tower menu.
+msg.rune_next = The rune forge plans are packed away: the tower comes back in the next mission.
+msg.rune_back = The rune forge plans are back - you may build the tower once more.
+msg.dwarf_slain = A dwarf falls. +{0} gold.
+info.dwarf = Guards the hoard of a dwarf cave
+info.dwarf_neutral = Neutral: fights whoever comes close, never leaves the cave.
+tile.rune_chest = Dwarven rune chest
+tile.rune_chest_info = Claim it: the plans of a rune forge tower (one free build).