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/main.rs

@@ -23,6 +23,7 @@ mod sfx;
 mod tracks;
 mod traps;
 mod sound;
+mod spells;
 mod touch;
 mod ui;
 mod xm;
@@ -118,6 +119,7 @@ fn world_action(app: &mut App, ui: &mut Ui, r3d: &R3d, inp: &mut Input, mx: f32,
         (Tool::Summon, _) => app.game.summon(wx, wy),
         (Tool::Tower(k), Some((x, z))) => app.game.build_tower(k, x, z),
         (Tool::Trap(k), Some((x, z))) => app.game.place_trap(k, x, z),
+        (Tool::Spell(sp), _) => app.game.cast(sp, wx, wy),
         _ => {}
     }
 }
@@ -1049,6 +1051,7 @@ fn draw_frame(app: &mut App, gfx: &Gfx, r3d: &mut R3d, ui: &mut Ui, menu: &mut M
         Tool::Steel => (Color::new(0.75, 0.8, 0.9, 1.0), None),
         Tool::Tower(_) => (Color::new(1.0, 0.6, 0.25, 1.0), None),
         Tool::Trap(_) => (Color::new(1.0, 0.4, 0.3, 1.0), None),
+        Tool::Spell(sp) => (Color::new(0.8, 0.55, 1.0, 1.0), Some((ground.x, ground.y, sp.radius().max(0.6)))),
     };
     let ov = Overlay {
         hover: if over_ui { None } else { hover_tile.filter(|&(x, z)| inb(x, z)) },