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/render3d.rs

@@ -200,6 +200,8 @@ uniform float LightCount;
 void main() {
     vec4 tex = texture2D(Texture, uv);
     if (tex.a < 0.5) discard;
+    // see-through units (invisibility): every other pixel left out
+    if (color.a < 0.99 && mod(floor(gl_FragCoord.x) + floor(gl_FragCoord.y), 2.0) < 1.0) discard;
     vec3 n = normalize(wnorm);
     vec3 light = color.rgb;
     for (int i = 0; i < 24; i++) {
@@ -1200,12 +1202,18 @@ impl R3d {
                 std::mem::swap(&mut u0, &mut u1);
             }
             let a = amb(e.x, e.y) * if sleeping || e.captive == 1 { 0.6 } else { 1.0 };
+            // spell looks: stone grey, plague green, see-through when invisible
+            let alpha = if e.invis > 0.0 { 0.35 } else { 1.0 };
             let c = if white {
                 [255, 255, 255, 255]
             } else if e.captive == 3 {
                 col(vec3(a * 1.2, a * 0.55, a * 0.55), 1.0)
+            } else if e.shield > 0.0 {
+                col(vec3(a * 0.85, a * 0.85, a * 0.8), alpha)
+            } else if e.plague > 0.0 {
+                col(vec3(a * 0.75, a * 1.15, a * 0.6), alpha)
             } else {
-                col(Vec3::splat(a), 1.0)
+                col(Vec3::splat(a), alpha)
             };
             // pulled towards the camera: unchanged on screen (orthographic), but walls behind
             // the unit no longer cut into the sprite