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 levels should have ambient floating particles (dust motes, fine
-sand) drifting across the screen to sell the atmosphere. Could tie into the
-existing wind system — particles drift with the wind direction. Should be
-subtle, low-alpha, and layered behind/in front of the player for depth.
+## ~~Mars Surface atmosphere particles~~ ✓
+Implemented: `particle_emit_atmosphere_dust()` emits ambient dust motes each
+frame on Mars Surface levels (keyed on `PARALLAX_STYLE_MARS`). Three
+sub-layers for depth: large slow "far" motes, small quick "near" specks,
+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,