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/particles.rs

@@ -76,7 +76,7 @@ pub fn fx_pos(f: &Fx, p: f32) -> (f32, f32, f32) {
 }
 
 impl Game {
-    fn part(&mut self, kind: u8, (x, y, h): (f32, f32, f32), (vx, vy, vh): (f32, f32, f32), max: f32, col: u32, size: f32) {
+    pub(crate) fn part(&mut self, kind: u8, (x, y, h): (f32, f32, f32), (vx, vy, vh): (f32, f32, f32), max: f32, col: u32, size: f32) {
         if self.parts.len() < PARTS_MAX {
             self.parts.push(Part { kind, x, y, h, vx, vy, vh, t: 0.0, max, col, size });
         }