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>

test/README

@@ -0,0 +1,9 @@
+Unit tests (PlatformIO Unit Testing / Unity) go here.
+
+Planned first test: EL512Panel::encodeRow() - check that HS is high for exactly
+256 clocks per visible row, VS appears only on the last blanking line, and that
+even/odd columns land on VID/TVID.
+
+Run with:  pio test -e esp32dev
+
+More info: https://docs.platformio.org/en/latest/advanced/unit-testing/index.html