Eight new spells: seer's eye, cave-in, haste, stoneskin, gold rain, invisibility, plague, quake

- researched in the library (and unlocked in the campaign), in the spells menu with own icons and a radius under the pointer
- haste speeds up everything a creature does, stoneskin halves damage, invisible creatures are ignored by enemies and towers until they attack (drawn see-through), plague spreads among enemies, quake hurts and stuns enemies only, cave-in turns floor back into earth
- particles for every spell

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

src/campaign.rs

@@ -10,7 +10,7 @@ use serde::{Deserialize, Serialize};
 // From which mission (0-based) on each tool is available (same order as ui::TOOLS):
 // dig, hand, treasury, lair, farm, training, library, sell, flag, lightning, heal, imp,
 // torture, temple, arrow tower, cannon tower, magic tower, bridge, aerie, steel
-pub const TOOL_UNLOCK: [usize; 32] = [0, 0, 0, 0, 0, 2, 5, 0, 0, 3, 5, 0, 8, 12, 3, 7, 12, 1, 8, 17, 0, 7, 7, 10, 12, 12, 7, 16, 16, 7, 10, 16];
+pub const TOOL_UNLOCK: [usize; 40] = [0, 0, 0, 0, 0, 2, 5, 0, 0, 3, 5, 0, 8, 12, 3, 7, 12, 1, 8, 17, 0, 7, 7, 10, 12, 12, 7, 16, 16, 7, 10, 16, 9, 13, 6, 10, 14, 17, 19, 22];
 
 pub struct FortDef {
     pub at: (f32, f32), // centre as a fraction of the map size