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).

platformio.ini

@@ -0,0 +1,56 @@
+; Root PlatformIO configuration for LoRa KISS modem (20 boards)
+; Board-specific environments are in hardware/<vendor>/<board>/platformio.ini
+; SoC shared settings are in soc/<soc>/platformio.ini
+
+[platformio]
+default_envs =
+    heltec_v3,
+    rak_rak4631,
+    heltec_v2
+
+extra_configs =
+    soc/esp32/platformio.ini
+    soc/esp32s3/platformio.ini
+    soc/nrf52/platformio.ini
+    hardware/heltec/t114/platformio.ini
+    hardware/heltec/ct62/platformio.ini
+    hardware/heltec/e213/platformio.ini
+    hardware/heltec/e290/platformio.ini
+    hardware/heltec/mesh_solar/platformio.ini
+    hardware/heltec/t190/platformio.ini
+    hardware/heltec/tracker/platformio.ini
+    hardware/heltec/tracker_v2/platformio.ini
+    hardware/heltec/v2/platformio.ini
+    hardware/heltec/v3/platformio.ini
+    hardware/heltec/v4/platformio.ini
+    hardware/lilygo/t_beam_1w/platformio.ini
+    hardware/lilygo/t_beam_sx1262/platformio.ini
+    hardware/lilygo/t_beam_sx1276/platformio.ini
+    hardware/lilygo/t_beam_supreme/platformio.ini
+    hardware/seeed/xiao_s3_wio_sx1262/platformio.ini
+    hardware/rak/rak11310/platformio.ini
+    hardware/rak/rak3112/platformio.ini
+    hardware/rak/rak3401/platformio.ini
+    hardware/rak/rak3x72/platformio.ini
+    hardware/rak/rak4631/platformio.ini
+
+; ------------------------------------------------------------------
+; Base environment — all board envs extend this.
+; Provides: RadioLib dependency, default LoRa parameters.
+; ------------------------------------------------------------------
+[env:base]
+lib_deps =
+    jgromes/RadioLib@^6.6.0
+
+monitor_speed = 115200
+
+build_flags =
+    ; Default LoRa radio parameters — override per-board as needed
+    -DLORA_FREQ_KHZ=869525UL
+    -DLORA_BW_HZ=125000UL
+    -DLORA_SF=7
+    -DLORA_CR=5
+    -DLORA_POWER_DBM=14
+    -DLORA_SYNCWORD=0x34
+    ; KISS serial baud rate
+    -DKISS_BAUD=115200