Add per-level wind atmosphere property

WIND directive in .lvl files sets a constant horizontal force (px/s^2)
that pushes entities, projectiles, and particles. Positive is rightward.

Wind is applied as acceleration in physics_update() (halved on ground),
directly to projectile and particle velocities, and as a gentle position
drift on flyers. Entities with gravity_scale=0 (drones, spacecraft) are
unaffected. Levels default to no wind when the directive is absent.

DESIGN.md

@@ -48,7 +48,7 @@ Each level defines its own atmosphere, affecting gameplay feel:
 | `MUSIC`        | Level music track                           | assets/music/level1.ogg |
 | `PALETTE`      | Color mood (warm, cold, toxic, void)        | tint/filter values      |
 
-Already implemented: `GRAVITY`, `BG_COLOR`, `MUSIC`, `PARALLAX_FAR`, `PARALLAX_NEAR` (all per-level). Parallax backgrounds are procedurally generated (starfield + nebula) when no image path is specified.
+Already implemented: `GRAVITY`, `WIND`, `BG_COLOR`, `MUSIC`, `PARALLAX_FAR`, `PARALLAX_NEAR` (all per-level). Parallax backgrounds are procedurally generated (starfield + nebula) when no image path is specified.
 
 ---
 
@@ -118,11 +118,10 @@ adding a new def. See `src/game/projectile.h` for the full definition.
 ## Levels
 
 ### Format (.lvl)
-Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `BG_COLOR`, `MUSIC`, `PARALLAX_FAR`, `PARALLAX_NEAR`, `TILEDEF`, `ENTITY`, `LAYER`
+Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `WIND`, `BG_COLOR`, `MUSIC`, `PARALLAX_FAR`, `PARALLAX_NEAR`, `TILEDEF`, `ENTITY`, `EXIT`, `LAYER`
 
 **Needed additions:**
-- `EXIT <tile_x> <tile_y> <next_level>` — Level exit zone
-- `WIND`, `STORM`, `DRAG` — Atmosphere settings
+- `STORM`, `DRAG` — Remaining atmosphere settings
 
 ### Level Ideas
 
@@ -166,7 +165,8 @@ Current directives: `TILESET`, `SIZE`, `SPAWN`, `GRAVITY`, `BG_COLOR`, `MUSIC`,
 
 ### Medium Priority
 - [x] In-game level editor (tile/entity placement, save/load, test play)
-- [ ] Wind / drag atmosphere properties
+- [x] Wind atmosphere property (`WIND` directive, affects all entities/particles/projectiles)
+- [ ] Drag atmosphere property
 - [x] Parallax scrolling backgrounds (procedural stars + nebula, or from image files)
 - [x] Per-level background color (`BG_COLOR` directive)
 - [x] Music playback per level (`MUSIC` directive)

src/engine/particle.c

@@ -82,6 +82,7 @@ void particle_emit(const ParticleBurst *b) {
 
 void particle_update(float dt) {
     float gravity = physics_get_gravity();
+    float wind    = physics_get_wind();
 
     for (int i = 0; i < MAX_PARTICLES; i++) {
         Particle *p = &s_particles[i];
@@ -96,6 +97,11 @@ void particle_update(float dt) {
         /* Apply gravity */
         p->vel.y += gravity * p->gravity_scale * dt;
 
+        /* Apply wind (reuse gravity_scale as environmental-force scale) */
+        if (wind != 0.0f && p->gravity_scale > 0.0f) {
+            p->vel.x += wind * p->gravity_scale * dt;
+        }
+
         /* Apply drag */
         if (p->drag > 0) {
             float factor = 1.0f - p->drag * dt;

src/engine/physics.c

@@ -4,9 +4,11 @@
 #include <math.h>
 
 static float s_gravity = DEFAULT_GRAVITY;
+static float s_wind    = 0.0f;
 
 void physics_init(void) {
     s_gravity = DEFAULT_GRAVITY;
+    s_wind    = 0.0f;
 }
 
 void physics_set_gravity(float gravity) {
@@ -17,6 +19,14 @@ float physics_get_gravity(void) {
     return s_gravity;
 }
 
+void physics_set_wind(float wind) {
+    s_wind = wind;
+}
+
+float physics_get_wind(void) {
+    return s_wind;
+}
+
 static void resolve_tilemap_x(Body *body, const Tilemap *map) {
     if (!map) return;
 
@@ -102,6 +112,12 @@ void physics_update(Body *body, float dt, const Tilemap *map) {
     /* Apply gravity */
     body->vel.y += s_gravity * body->gravity_scale * dt;
 
+    /* Apply wind — halved on ground (friction counteracts) */
+    if (s_wind != 0.0f && body->gravity_scale > 0.0f) {
+        float wind_factor = body->on_ground ? 0.5f : 1.0f;
+        body->vel.x += s_wind * body->gravity_scale * wind_factor * dt;
+    }
+
     /* Clamp fall speed */
     if (body->vel.y > MAX_FALL_SPEED) {
         body->vel.y = MAX_FALL_SPEED;

src/engine/physics.h

@@ -25,6 +25,10 @@ void physics_init(void);
 void  physics_set_gravity(float gravity);
 float physics_get_gravity(void);
 
+/* Set / get the current horizontal wind force (pixels/s^2, +=right) */
+void  physics_set_wind(float wind);
+float physics_get_wind(void);
+
 /* Move body, apply gravity, resolve against tilemap */
 void physics_update(Body *body, float dt, const Tilemap *map);
 

src/engine/tilemap.c

@@ -85,6 +85,8 @@ bool tilemap_load(Tilemap *map, const char *path, SDL_Renderer *renderer) {
             map->player_spawn = vec2(sx * TILE_SIZE, sy * TILE_SIZE);
         } else if (strncmp(line, "GRAVITY ", 8) == 0) {
             sscanf(line + 8, "%f", &map->gravity);
+        } else if (strncmp(line, "WIND ", 5) == 0) {
+            sscanf(line + 5, "%f", &map->wind);
         } else if (strncmp(line, "TILEDEF ", 8) == 0) {
             int id, tx, ty;
             uint32_t flags;

src/engine/tilemap.h

@@ -50,6 +50,7 @@ typedef struct Tilemap {
     char         tileset_path[ASSET_PATH_MAX]; /* tileset file path */
     Vec2         player_spawn;
     float        gravity;           /* level gravity (px/s^2), 0 = use default */
+    float        wind;              /* horizontal wind force (px/s^2), +=right  */
     char         music_path[ASSET_PATH_MAX]; /* level music file path          */
     SDL_Color    bg_color;          /* background clear color                  */
     bool         has_bg_color;      /* true if BG_COLOR was set                */

src/game/editor.c

@@ -450,6 +450,8 @@ static bool save_tilemap(const Tilemap *map, const char *path) {
 
     if (map->gravity > 0)
         fprintf(f, "GRAVITY %.0f\n", map->gravity);
+    if (map->wind != 0.0f)
+        fprintf(f, "WIND %.0f\n", map->wind);
     if (map->has_bg_color)
         fprintf(f, "BG_COLOR %d %d %d\n", map->bg_color.r, map->bg_color.g, map->bg_color.b);
     if (map->music_path[0])

src/game/enemy.c

@@ -170,6 +170,13 @@ static void flyer_update(Entity *self, float dt, const Tilemap *map) {
     float bob_offset = sinf(fd->bob_timer * FLYER_BOB_SPD) * FLYER_BOB_AMP;
     body->pos.y = fd->base_y + bob_offset;
 
+    /* Wind drifts flyers (they bypass physics_update).
+     * Apply as gentle position offset matching first-frame physics. */
+    float wind = physics_get_wind();
+    if (wind != 0.0f) {
+        body->pos.x += wind * dt * dt;
+    }
+
     /* Chase player if in range */
     Entity *player = find_player(s_flyer_em);
     if (player && player->active && !(player->flags & ENTITY_DEAD)) {

src/game/level.c

@@ -57,6 +57,9 @@ static bool level_setup(Level *level) {
         physics_set_gravity(DEFAULT_GRAVITY);
     }
 
+    /* Apply level wind (0 = no wind) */
+    physics_set_wind(level->map.wind);
+
     /* Apply level background color */
     if (level->map.has_bg_color) {
         renderer_set_clear_color(level->map.bg_color);

src/game/levelgen.c

@@ -1631,6 +1631,10 @@ bool levelgen_dump_lvl(const Tilemap *map, const char *path) {
         fprintf(f, "GRAVITY %.0f\n", map->gravity);
     }
 
+    if (map->wind != 0.0f) {
+        fprintf(f, "WIND %.0f\n", map->wind);
+    }
+
     if (map->has_bg_color) {
         fprintf(f, "BG_COLOR %d %d %d\n",
                 map->bg_color.r, map->bg_color.g, map->bg_color.b);

src/game/projectile.c

@@ -204,6 +204,12 @@ static void projectile_update(Entity *self, float dt, const Tilemap *map) {
         if (body->vel.y > MAX_FALL_SPEED) body->vel.y = MAX_FALL_SPEED;
     }
 
+    /* ── Apply wind ─────────────────────────── */
+    float wind = physics_get_wind();
+    if (wind != 0.0f) {
+        body->vel.x += wind * dt;
+    }
+
     /* ── Move ────────────────────────────────── */
     body->pos.x += body->vel.x * dt;
     body->pos.y += body->vel.y * dt;