Add hit markers and metal explosion for turrets

Turrets now emit orange-white spark particles when taking non-lethal
damage, giving clear visual feedback on hits. On death, turrets get a
dedicated metal explosion effect (shrapnel, hot sparks, flash, smoke)
instead of the generic death puff, with stronger screen shake.

Closes #14

src/engine/particle.c

@@ -530,6 +530,124 @@ void particle_emit_wall_slide_dust(Vec2 pos, int wall_dir) {
     particle_emit(&dust);
 }
 
+void particle_emit_hit_sparks(Vec2 pos) {
+    /* Bright orange-white sparks — fast, short-lived, spray outward */
+    ParticleBurst sparks = {
+        .origin    = pos,
+        .count     = 8,
+        .speed_min = 60.0f,
+        .speed_max = 180.0f,
+        .life_min  = 0.08f,
+        .life_max  = 0.2f,
+        .size_min  = 1.0f,
+        .size_max  = 2.0f,
+        .spread    = (float)M_PI,
+        .direction = 0,
+        .drag      = 3.0f,
+        .gravity_scale = 0.4f,
+        .color     = {255, 200, 100, 255},  /* orange-white */
+        .color_vary = true,
+    };
+    particle_emit(&sparks);
+
+    /* Brief white flash at impact point */
+    ParticleBurst flash = {
+        .origin    = pos,
+        .count     = 3,
+        .speed_min = 5.0f,
+        .speed_max = 15.0f,
+        .life_min  = 0.03f,
+        .life_max  = 0.06f,
+        .size_min  = 2.0f,
+        .size_max  = 3.5f,
+        .spread    = (float)M_PI,
+        .direction = 0,
+        .drag      = 8.0f,
+        .gravity_scale = 0.0f,
+        .color     = {255, 255, 230, 255},  /* bright white */
+        .color_vary = false,
+    };
+    particle_emit(&flash);
+}
+
+void particle_emit_metal_explosion(Vec2 pos) {
+    /* Metal shrapnel — fast grey/silver chunks flying outward */
+    ParticleBurst shrapnel = {
+        .origin    = pos,
+        .count     = 16,
+        .speed_min = 60.0f,
+        .speed_max = 200.0f,
+        .life_min  = 0.2f,
+        .life_max  = 0.6f,
+        .size_min  = 1.5f,
+        .size_max  = 3.5f,
+        .spread    = (float)M_PI,
+        .direction = 0,
+        .drag      = 2.0f,
+        .gravity_scale = 0.5f,
+        .color     = {180, 180, 190, 255},  /* silver-grey */
+        .color_vary = true,
+    };
+    particle_emit(&shrapnel);
+
+    /* Hot orange sparks — electrical/mechanical innards */
+    ParticleBurst sparks = {
+        .origin    = pos,
+        .count     = 10,
+        .speed_min = 40.0f,
+        .speed_max = 150.0f,
+        .life_min  = 0.15f,
+        .life_max  = 0.35f,
+        .size_min  = 1.0f,
+        .size_max  = 2.5f,
+        .spread    = (float)M_PI,
+        .direction = 0,
+        .drag      = 2.5f,
+        .gravity_scale = 0.3f,
+        .color     = {255, 160, 40, 255},   /* hot orange */
+        .color_vary = true,
+    };
+    particle_emit(&sparks);
+
+    /* Bright white flash at center — brief pop */
+    ParticleBurst flash = {
+        .origin    = pos,
+        .count     = 5,
+        .speed_min = 5.0f,
+        .speed_max = 20.0f,
+        .life_min  = 0.04f,
+        .life_max  = 0.08f,
+        .size_min  = 3.0f,
+        .size_max  = 5.0f,
+        .spread    = (float)M_PI,
+        .direction = 0,
+        .drag      = 8.0f,
+        .gravity_scale = 0.0f,
+        .color     = {255, 240, 200, 255},  /* bright flash */
+        .color_vary = false,
+    };
+    particle_emit(&flash);
+
+    /* Smoke cloud — slower, lingers */
+    ParticleBurst smoke = {
+        .origin    = pos,
+        .count     = 8,
+        .speed_min = 15.0f,
+        .speed_max = 50.0f,
+        .life_min  = 0.3f,
+        .life_max  = 0.7f,
+        .size_min  = 2.5f,
+        .size_max  = 4.5f,
+        .spread    = (float)M_PI,
+        .direction = 0,
+        .drag      = 3.5f,
+        .gravity_scale = -0.1f,  /* floats up */
+        .color     = {120, 120, 130, 180},  /* dark smoke */
+        .color_vary = true,
+    };
+    particle_emit(&smoke);
+}
+
 /* Spawn a single dust mote with the given visual properties. */
 static void spawn_dust_mote(Vec2 pos, Vec2 vel,
                             float life_min, float life_max,

src/engine/particle.h

@@ -89,6 +89,12 @@ void particle_emit_muzzle_flash(Vec2 pos, Vec2 shoot_dir);
 /* Wall slide dust (small puffs while scraping against a wall) */
 void particle_emit_wall_slide_dust(Vec2 pos, int wall_dir);
 
+/* Metal hit sparks (turret/machine takes non-lethal damage) */
+void particle_emit_hit_sparks(Vec2 pos);
+
+/* Metal explosion (turret/machine death — shrapnel + flash) */
+void particle_emit_metal_explosion(Vec2 pos);
+
 /* Ambient atmosphere dust (call each frame for Mars Surface levels).
  * Spawns subtle dust motes around the camera viewport that drift with wind.
  * cam_pos = camera top-left world position, vp = viewport size in pixels. */