perf(box): cache Lire icons in PSRAM

The reading game referenced icons by SD file path, so LVGL re-read and decoded
each .bin from the slow microSD on every draw band (16-line buffer) and every
frame, with no image cache — hence the laggy display. Read each .bin once into
PSRAM and hand LVGL an in-RAM image descriptor: no per-band SD reads, and
repeated words are instant.

main/modes/lire.c

@@ -139,17 +139,68 @@ static void play_word(int idx)
     lisael_aac_play_file(p);
 }
 
-// A word's picture from the SD (or a coloured fallback with its first letter).
+// --- PSRAM icon cache --------------------------------------------------------
+// Referencing icons by SD file path ("S:/…") made LVGL re-read+decode each .bin
+// from the slow microSD on every draw band (the draw buffer is only 16 lines)
+// AND every frame, with no image cache (LV_CACHE_DEF_SIZE=0) — hence the laggy
+// display. Instead we read each .bin ONCE into PSRAM and hand LVGL an in-RAM
+// image descriptor: no per-band SD reads ever, and repeated words are instant.
+typedef struct {
+    char           id[28];
+    void          *buf;      // the whole .bin in PSRAM (NULL = known-missing)
+    lv_image_dsc_t dsc;
+} lire_icon_t;
+
+static lire_icon_t *s_icons;   // PSRAM, one entry per distinct id seen
+static int          s_icons_n;
+
+static const lv_image_dsc_t *icon_dsc(const char *id)
+{
+    for (int i = 0; i < s_icons_n; i++) {
+        if (!strcmp(s_icons[i].id, id)) {
+            return s_icons[i].buf ? &s_icons[i].dsc : NULL;
+        }
+    }
+    if (!s_icons) {
+        s_icons = heap_caps_calloc(LIRE_MAX_WORDS, sizeof(lire_icon_t), MALLOC_CAP_SPIRAM);
+        if (!s_icons) return NULL;
+    }
+    if (s_icons_n >= LIRE_MAX_WORDS) return NULL;
+    lire_icon_t *c = &s_icons[s_icons_n++];   // also negatively caches misses
+    snprintf(c->id, sizeof(c->id), "%s", id);
+    c->buf = NULL;
+
+    char p[80];
+    snprintf(p, sizeof(p), "/sdcard/lire/img/%s.bin", id);
+    FILE *f = fopen(p, "rb");
+    if (!f) return NULL;
+    fseek(f, 0, SEEK_END);
+    long sz = ftell(f);
+    fseek(f, 0, SEEK_SET);
+    if (sz <= (long)sizeof(lv_image_header_t) || sz > 256 * 1024) { fclose(f); return NULL; }
+    uint8_t *buf = heap_caps_malloc((size_t)sz, MALLOC_CAP_SPIRAM);
+    if (!buf) { fclose(f); return NULL; }
+    size_t rd = fread(buf, 1, (size_t)sz, f);
+    fclose(f);
+    if (rd != (size_t)sz) { heap_caps_free(buf); return NULL; }
+
+    // The LVGL .bin layout is lv_image_header_t followed by pixel data, so the
+    // PSRAM copy doubles as a variable-image descriptor (same bytes LVGL would
+    // read from the file — which already renders correctly here).
+    lv_memcpy(&c->dsc.header, buf, sizeof(lv_image_header_t));
+    c->dsc.data      = buf + sizeof(lv_image_header_t);
+    c->dsc.data_size = (uint32_t)(sz - (long)sizeof(lv_image_header_t));
+    c->buf           = buf;
+    return &c->dsc;
+}
+
+// A word's picture (from the PSRAM cache) or a coloured fallback with its initial.
 static lv_obj_t *make_word_icon(lv_obj_t *parent, int idx, int px)
 {
-    char real[80];
-    snprintf(real, sizeof(real), "/sdcard/lire/img/%s.bin", s_words[idx].id);
-    struct stat st;
-    if (stat(real, &st) == 0 && st.st_size > 0) {
-        char src[88];
-        snprintf(src, sizeof(src), "S:/sdcard/lire/img/%s.bin", s_words[idx].id);
+    const lv_image_dsc_t *d = icon_dsc(s_words[idx].id);
+    if (d) {
         lv_obj_t *img = lv_image_create(parent);
-        lv_image_set_src(img, src);
+        lv_image_set_src(img, d);
         lv_obj_set_size(img, px, px);
         lv_image_set_inner_align(img, LV_IMAGE_ALIGN_CONTAIN);
         return img;