Add Mars surface atmosphere particels

TODO.md

@@ -145,11 +145,15 @@ which defers a level load into MODE_PLAY on the next frame. Tears down the
 current mode (editor or play), loads the selected `.lvl` file, and seeds
 `s_edit_path` so pressing E opens the same level in the editor.
 
-## Mars Surface atmosphere particles
+## ~~Mars Surface atmosphere particles~~ ✓
-Mars Surface levels should have ambient floating particles (dust motes, fine
+Implemented: `particle_emit_atmosphere_dust()` emits ambient dust motes each
-sand) drifting across the screen to sell the atmosphere. Could tie into the
+frame on Mars Surface levels (keyed on `PARALLAX_STYLE_MARS`). Three
-existing wind system — particles drift with the wind direction. Should be
+sub-layers for depth: large slow "far" motes, small quick "near" specks,
-subtle, low-alpha, and layered behind/in front of the player for depth.
+and occasional interior spawns to prevent edge seams. All particles drift
+with the wind system via `gravity_scale` — wind pushes them across the
+viewport. Reddish-tan color palette with per-particle variation. Low drag,
+long lifetimes (3-7 s), subtle alpha fade. ~2-3 particles/frame at 60 Hz,
+well within the 1024-particle pool budget.
 
 ## Skip spacecraft transition for non-surface levels
 The spacecraft fly-in animation should only play on surface levels (moon01,

src/engine/particle.c

@@ -529,3 +529,65 @@ void particle_emit_wall_slide_dust(Vec2 pos, int wall_dir) {
     };
     particle_emit(&dust);
 }
+
+/* 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,
+                            float size_min, float size_max,
+                            float drag, float gscale,
+                            uint8_t r, uint8_t g, uint8_t b, int vary) {
+    Particle *p = alloc_particle();
+    p->pos       = pos;
+    p->vel       = vel;
+    p->life      = randf_range(life_min, life_max);
+    p->max_life  = p->life;
+    p->size      = randf_range(size_min, size_max);
+    p->drag      = drag;
+    p->gravity_scale = gscale;
+    p->active    = true;
+    p->color.r   = clamp_u8(r + (int)randf_range(-vary, vary));
+    p->color.g   = clamp_u8(g + (int)randf_range(-vary, vary));
+    p->color.b   = clamp_u8(b + (int)randf_range(-vary, vary));
+    p->color.a   = 255; /* alpha applied during render from life ratio */
+}
+
+void particle_emit_atmosphere_dust(Vec2 cam_pos, Vec2 vp) {
+    /* Ambient Mars dust — subtle motes drifting across the viewport.
+     * Two sub-layers for depth: large slow "far" motes and small quick
+     * "near" specks. Wind carries them; gravity_scale controls how much
+     * environmental forces (wind + gravity) affect each particle.
+     * Particles spawn along the upwind viewport edge and drift inward;
+     * occasional interior spawns prevent a visible edge seam. */
+    float wind   = physics_get_wind();
+    float margin = 32.0f;
+    float dir    = (wind >= 0.0f) ? 1.0f : -1.0f; /* velocity sign */
+
+    /* Upwind edge X for the two edge-spawned layers */
+    float edge_far  = (wind >= 0.0f) ? cam_pos.x - margin
+                                     : cam_pos.x + vp.x + margin;
+    float edge_near = (wind >= 0.0f) ? cam_pos.x - margin * 0.5f
+                                     : cam_pos.x + vp.x + margin * 0.5f;
+
+    /* Far dust motes — large, slow, translucent (1/frame) */
+    spawn_dust_mote(
+        vec2(edge_far, cam_pos.y + randf() * vp.y),
+        vec2(dir * randf_range(8.0f, 25.0f), randf_range(-6.0f, 6.0f)),
+        4.0f, 7.0f, 1.5f, 3.0f, 0.3f, 0.08f,
+        180, 140, 100, 25);
+
+    /* Near dust specks — small, quicker, brighter (1/frame) */
+    spawn_dust_mote(
+        vec2(edge_near, cam_pos.y + randf() * vp.y),
+        vec2(dir * randf_range(15.0f, 40.0f), randf_range(-10.0f, 10.0f)),
+        2.5f, 5.0f, 0.8f, 1.5f, 0.2f, 0.12f,
+        200, 160, 120, 20);
+
+    /* Occasional interior spawn — prevents edge seam on calm wind */
+    if (rand() % 3 == 0) {
+        spawn_dust_mote(
+            vec2(cam_pos.x + randf() * vp.x, cam_pos.y + randf() * vp.y),
+            vec2(randf_range(-5.0f, 5.0f), randf_range(-8.0f, 3.0f)),
+            3.0f, 6.0f, 1.0f, 2.5f, 0.4f, 0.06f,
+            160, 130, 95, 25);
+    }
+}

src/engine/particle.h

@@ -89,4 +89,9 @@ 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);
 
+/* 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. */
+void particle_emit_atmosphere_dust(Vec2 cam_pos, Vec2 vp);
+
 #endif /* JNR_PARTICLE_H */

src/game/level.c

@@ -578,6 +578,15 @@ void level_update(Level *level, float dt) {
         }
     }
 
+    /* Emit ambient atmosphere dust on Mars Surface levels before the
+     * particle update pass so new motes get their first physics step
+     * this frame — consistent with other per-frame emitters. */
+    if (level->map.parallax_style == PARALLAX_STYLE_MARS) {
+        particle_emit_atmosphere_dust(
+            level->camera.pos,
+            level->camera.viewport);
+    }
+
     /* Update particles */
     particle_update(dt);