Cycle rain streams through the sprite pool for a sense of change

Each stream periodically swaps to a different one of the pooled streak
sprites on a staggered timer, so the columns keep changing rather than
sitting statically tiled — the rain reads as alive in the corner of the
eye. The pool stays at 5: each sprite is ~15 KB of internal RAM and more
starves esp-hosted's internal-RAM task stacks into a boot loop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LKPbR6DY2JygHbyLjxm7Uu

firmware/main/face.c

@@ -32,7 +32,9 @@ LV_FONT_DECLARE(font_rage_64);
 #define SPRITE_GLYPHS  12                      /* streak length in a sprite */
 #define SPRITE_W       24                      /* sprite width (glyph ~22px) */
 #define SPRITE_H       (SPRITE_GLYPHS * CELL_H)
-#define SPRITE_POOL    5                        /* distinct pre-rendered streaks */
+#define SPRITE_POOL    5                        /* distinct pre-rendered streaks. Capped at 5:
+                                                  * each ~15 KB in internal RAM, and more starves
+                                                  * esp-hosted's internal-RAM task stacks (boot loop). */
 #define RAIN_COLS (SCREEN / RAIN_CELL)
 #define TICK_MS 33
 
@@ -89,6 +91,7 @@ static const char *GLYPHS[] = {
 typedef struct {
     lv_obj_t *img;   /* lv_image showing one of the pool sprites */
     int sprite;      /* which pool sprite (index into sprite_dsc) */
+    int flip_in;     /* ticks until this stream swaps to another pool sprite */
     int x, y, speed;
     bool alive;
 } stream_t;
@@ -174,6 +177,7 @@ static void stream_spawn(stream_t *s, int speed)
     s->speed = speed + esp_random() % 4;
     s->y = -SPRITE_H;
     s->sprite = esp_random() % SPRITE_POOL;
+    s->flip_in = 12 + esp_random() % 28;
     if (s->img == NULL) {
         s->img = lv_image_create(F.rain_layer);
     }
@@ -218,6 +222,11 @@ static void rain_tick(void)
         alive++;
         s->y += s->speed * 2;              /* cheap blit of a pre-rendered sprite */
         lv_obj_set_pos(s->img, s->x, s->y);
+        if (--s->flip_in <= 0) {           /* swap to a different streak sprite: the "flip" */
+            s->sprite = (s->sprite + 1 + esp_random() % (SPRITE_POOL - 1)) % SPRITE_POOL;
+            lv_image_set_src(s->img, &sprite_dsc[s->sprite]);
+            s->flip_in = 12 + esp_random() % 28;
+        }
         if (s->y > SCREEN) {
             if (alive > target) {
                 stream_hide(s);