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).

PROJECT.md

@@ -146,22 +146,26 @@ Not all bandwidths are legal for all frequency bands — consult local regulatio
 
 ## Project Structure
 
+Three-tier configuration hierarchy: base + SoC + board. Pin definitions are compile-time
+macros in board configs (no separate pins.h).
+
 ```text
 loramodem/
-├── platformio.ini          # Board environments and build configuration
-├── PROJECT.md              # This file
-├── src/
-│   ├── main.cpp            # Entry point; Arduino setup() and loop()
-│   ├── kiss.h / kiss.cpp   # KISS frame encoder/decoder
-│   ├── radio.h / radio.cpp # RadioLib wrapper (init, TX, RX, config)
-│   └── config.h            # Default radio parameters and pin definitions
-├── include/                # Shared headers (if needed)
-├── lib/                    # Local libraries (if needed)
-├── variants/               # Board-specific hardware definitions
-│   ├── heltec_wifi_lora_32_v3/pins.h
-│   ├── rak4631/pins.h
-│   └── [other boards]/pins.h
-└── test/                   # PlatformIO Unity test suite
+├── platformio.ini                    # [platformio] + [env:base]
+├── soc/                              # SoC shared configs
+│   ├── esp32/platformio.ini
+│   ├── esp32s3/platformio.ini
+│   └── nrf52/platformio.ini
+├── hardware/                         # 20 board configs
+│   ├── heltec/   (11 boards)
+│   ├── lilygo/   (4 boards)
+│   ├── seeed/    (1 board)
+│   └── rak/      (4 boards)
+└── src/
+    ├── main.cpp            # Arduino setup()/loop() — calls C APIs
+    ├── kiss.h / kiss.c     # KISS protocol — C99
+    ├── radio.h / radio.cpp # RadioLib wrapper — C++ (extern "C" API)
+    └── config.h            # Compile-time defaults + pin validation
 ```
 
 ## Building
@@ -171,7 +175,9 @@ Prerequisites: [PlatformIO](https://platformio.org/) CLI or IDE extension.
 **Build for a specific board:**
 
 ```sh
-pio run -e heltec_wifi_lora_32_v3
+pio run -e heltec_v3
+pio run -e rak_rak4631
+pio run -e lilygo_t_beam_1w
 ```
 
 **Upload to a connected board:**