Fix PlatformIO build; document ESPHome component and ring-buffer driver

- el512_panel.cpp: esp_cpu_get_cycle_count() exists only in ESP-IDF 5;
  use cpu_hal_get_cycle_count() on 4.4 (Arduino 2.x), which broke both
  PlatformIO environments.
- README: ring-buffer refresh, tear-free status, D1 mini ESP32, new layout
  (components/el512, esphome/, home_assistant/), ESPHome setup, Still Alive
  paths, and the Wi-Fi blanking findings with wiring advice.
- docs/README: ring, lookup table, double buffer, health counters; tear-free
  and memory items done; Wi-Fi blanking and boot blanking as open items.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/el512/el512_panel.cpp

@@ -2,7 +2,7 @@
 
 #include <driver/gpio.h>
 #include <esp_attr.h>
-#include <esp_cpu.h>
+#include <esp_idf_version.h>
 #include <esp_heap_caps.h>
 #include <esp_intr_alloc.h>
 #include <esp_rom_gpio.h>
@@ -19,6 +19,14 @@
 #else
 #include <driver/periph_ctrl.h>
 #endif
+// CPU cycle counter: ESP-IDF 5 (ESPHome) and 4.4 (Arduino 2.x / PlatformIO).
+#if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(5, 0, 0)
+#include <esp_cpu.h>
+#define EL512_CYCLES() esp_cpu_get_cycle_count()
+#else
+#include <hal/cpu_hal.h>
+#define EL512_CYCLES() cpu_hal_get_cycle_count()
+#endif
 #if __has_include(<esp32/rom/lldesc.h>)
 #include <esp32/rom/lldesc.h>
 #else
@@ -227,7 +235,7 @@ void IRAM_ATTR EL512Panel::fillChunk(uint16_t chunk) {
 
 void IRAM_ATTR EL512Panel::isr(void* arg) {
   auto* self = static_cast<EL512Panel*>(arg);
-  const uint32_t now = esp_cpu_get_cycle_count();
+  const uint32_t now = EL512_CYCLES();
   if (self->_lastIsrCycle != 0 && now - self->_lastIsrCycle > self->_maxGap) self->_maxGap = now - self->_lastIsrCycle;
   self->_lastIsrCycle = now;
   self->_refills = self->_refills + 1;