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).

src/config.h

@@ -0,0 +1,51 @@
+#pragma once
+
+/* Default LoRa parameters — override per-board in hardware/.../platformio.ini */
+#ifndef LORA_FREQ_KHZ
+#  define LORA_FREQ_KHZ 869525UL   /* 869.525 MHz */
+#endif
+#ifndef LORA_BW_HZ
+#  define LORA_BW_HZ 125000UL   /* 125 kHz */
+#endif
+#ifndef LORA_SF
+#  define LORA_SF 7
+#endif
+#ifndef LORA_CR
+#  define LORA_CR 5   /* denominator: coding rate = 4/CR */
+#endif
+#ifndef LORA_POWER_DBM
+#  define LORA_POWER_DBM 14
+#endif
+#ifndef LORA_SYNCWORD
+#  define LORA_SYNCWORD 0x34   /* LoRa public */
+#endif
+
+#ifndef KISS_BAUD
+#  define KISS_BAUD 115200
+#endif
+
+/* Pin validation — boards must define all required pins via build_flags */
+#ifndef LORA_PIN_NSS
+#  error "LORA_PIN_NSS not defined — add to hardware/<vendor>/<board>/platformio.ini"
+#endif
+#ifndef LORA_PIN_RESET
+#  error "LORA_PIN_RESET not defined"
+#endif
+
+#if defined(LORA_CHIP_SX1276)
+#  ifndef LORA_PIN_DIO0
+#    error "LORA_PIN_DIO0 not defined (required for SX1276)"
+#  endif
+#else
+#  ifndef LORA_PIN_DIO1
+#    error "LORA_PIN_DIO1 not defined (required for SX1262/LR1110)"
+#  endif
+#  ifndef LORA_PIN_BUSY
+#    error "LORA_PIN_BUSY not defined (required for SX1262/LR1110)"
+#  endif
+#endif
+
+#if !defined(LORA_CHIP_SX1276) && !defined(LORA_CHIP_SX1262) &&             \
+    !defined(LORA_CHIP_LR1110)
+#  error "No LoRa chip defined — set LORA_CHIP_SX1276, LORA_CHIP_SX1262, or LORA_CHIP_LR1110"
+#endif