Set up PlatformIO project for EL512.256-H on ESP32

PlatformIO skeleton (esp32dev, Arduino, Adafruit GFX) with a local EL512
library: EL512Panel (1 bpp framebuffer, two-bit-mode row encoder, I2S DMA
refresh stub) and EL512_GFX (Adafruit_GFX subclass). Adds a test sketch,
pin map, README with wiring and timing, and moves the datasheet to
docs/datasheet/.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/main.cpp

@@ -0,0 +1,63 @@
+// Test sketch: bring up the EL512.256-H and draw a test pattern with Adafruit GFX.
+#include <Arduino.h>
+#include <EL512.h>
+
+#include "pins.h"
+
+static EL512Panel panel({
+    el512pins::VCLK,
+    el512pins::VID,
+    el512pins::TVID,
+    el512pins::HS,
+    el512pins::VS,
+    el512pins::ENABLE,
+    el512pins::LOWPWR_N,
+});
+static EL512_GFX display(panel);
+
+static void drawTestPattern() {
+  display.fillScreen(0);
+  display.drawRect(0, 0, display.width(), display.height(), 1);
+  display.drawLine(0, 0, display.width() - 1, display.height() - 1, 1);
+  display.drawLine(display.width() - 1, 0, 0, display.height() - 1, 1);
+
+  // Checkerboard strip to check VID/TVID column ordering.
+  for (int16_t y = 8; y < 24; y++)
+    for (int16_t x = 8; x < 136; x++)
+      if ((x ^ y) & 1) display.drawPixel(x, y, 1);
+
+  display.fillCircle(display.width() / 2, display.height() / 2, 40, 1);
+  display.setTextColor(1);
+  display.setTextSize(2);
+  display.setCursor(8, 32);
+  display.print("EL512.256-H");
+  display.setTextSize(1);
+  display.setCursor(8, 56);
+  display.printf("%ux%u  %.1f Hz", EL512Panel::WIDTH, EL512Panel::HEIGHT,
+                 EL512Panel::refreshHz());
+  display.display();
+}
+
+void setup() {
+  Serial.begin(115200);
+  Serial.println("EL512.256-H test");
+
+  if (!display.begin()) {
+    Serial.println("panel init failed (out of DMA memory?)");
+    while (true) delay(1000);
+  }
+  drawTestPattern();
+  Serial.printf("free heap: %u\n", ESP.getFreeHeap());
+}
+
+void loop() {
+  static uint32_t last = 0;
+  static uint32_t frames = 0;
+  if (millis() - last >= 1000) {
+    last = millis();
+    display.fillRect(8, 72, 120, 8, 0);
+    display.setCursor(8, 72);
+    display.printf("uptime %lus", (unsigned long)frames++);
+    display.display();
+  }
+}