Scaffold PlatformIO project with 20 board configs and C99/C++ source skeleton

Three-tier configuration hierarchy:
- [env:base] — RadioLib + default LoRa parameters
- [soc_esp32/esp32s3/nrf52] — platform + framework per SoC
- [env:board_name] — board-specific pins + chip selection

20 boards across 4 vendors:
- Heltec: 11 boards (T114, CT62, E213, E290, Mesh Solar, T190, Tracker,
  Tracker V2, V2, V3, V4)
- LilyGo: 4 boards (T-Beam 1W, sx1262, sx1276, supreme)
- Seeed: 1 board (Xiao S3 + Wio SX1262 with verified pins)
- RAK: 4 boards (RAK11310, RAK3112, RAK3401, RAK3x72, RAK4631)

Known/verified pins: Heltec V2/V3/V4, RAK4631, Seeed Xiao S3
FIXME pins: all others (placeholders for future research)

Source skeleton:
- config.h — compile-time defaults + pin validation (#error checks)
- kiss.h/c — KISS protocol implementation (C99)
- radio.h/cpp — RadioLib wrapper with C API (extern "C" boundary)
- main.cpp — Arduino entry point

All files pass pre-commit (prettier, markdownlint, YAML check).

soc/esp32/platformio.ini

@@ -0,0 +1,5 @@
+[soc_esp32]
+platform = espressif32
+framework = arduino
+build_flags =
+    -DARCH_ESP32

soc/esp32s3/platformio.ini

@@ -0,0 +1,5 @@
+[soc_esp32s3]
+platform = espressif32
+framework = arduino
+build_flags =
+    -DARCH_ESP32S3

soc/nrf52/platformio.ini

@@ -0,0 +1,5 @@
+[soc_nrf52]
+platform = nordicnrf52
+framework = arduino
+build_flags =
+    -DARCH_NRF52