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/kiss.h

@@ -0,0 +1,70 @@
+#pragma once
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/* KISS special bytes */
+#define KISS_FEND   0xC0u
+#define KISS_FESC   0xDBu
+#define KISS_TFEND  0xDCu
+#define KISS_TFESC  0xDDu
+
+/* Port assignments */
+#define KISS_PORT_DATA    0u
+#define KISS_PORT_QUALITY 1u
+#define KISS_PORT_CONFIG  2u
+
+/* Configuration command opcodes (port 2) */
+#define KISS_CMD_RESERVED     0x00u
+#define KISS_CMD_RES_OK       0x01u
+#define KISS_CMD_RES_ERROR    0x02u
+#define KISS_CMD_GET_RADIO    0x10u
+#define KISS_CMD_SET_RADIO    0x11u
+#define KISS_CMD_GET_FREQ     0x12u
+#define KISS_CMD_SET_FREQ     0x13u
+#define KISS_CMD_GET_BW       0x14u
+#define KISS_CMD_SET_BW       0x15u
+#define KISS_CMD_GET_SF       0x16u
+#define KISS_CMD_SET_SF       0x17u
+#define KISS_CMD_GET_CR       0x18u
+#define KISS_CMD_SET_CR       0x19u
+#define KISS_CMD_GET_POWER    0x1Au
+#define KISS_CMD_SET_POWER    0x1Bu
+#define KISS_CMD_GET_SYNCWORD 0x1Cu
+#define KISS_CMD_SET_SYNCWORD 0x1Du
+
+#define KISS_MAX_FRAME 256u
+
+typedef struct {
+    uint8_t  port;
+    uint8_t  data[KISS_MAX_FRAME];
+    uint16_t len;
+} kiss_frame_t;
+
+typedef enum {
+    KISS_STATE_IDLE,
+    KISS_STATE_IN_FRAME,
+    KISS_STATE_ESCAPE,
+} kiss_state_t;
+
+typedef struct {
+    kiss_state_t state;
+    uint8_t      buf[KISS_MAX_FRAME + 1u];   /* +1 for type byte */
+    uint16_t     len;
+} kiss_decoder_t;
+
+void kiss_decoder_init(kiss_decoder_t *dec);
+
+/* Feed one byte into the decoder. Returns true when a complete frame is
+   ready in *frame. frame must not be NULL when return value is checked. */
+bool kiss_decode(kiss_decoder_t *dec, uint8_t byte, kiss_frame_t *frame);
+
+/* Encode port+data into a KISS frame. Returns bytes written, or 0 on
+   overflow. */
+size_t kiss_encode(uint8_t port, const uint8_t *data, size_t len,
+                   uint8_t *dst, size_t dst_cap);
+
+/* Encode a 3-byte signal quality frame for port 1. Big-endian: int8 snr,
+   int16 rssi. */
+size_t kiss_encode_quality(int8_t snr, int16_t rssi, uint8_t *dst,
+                           size_t dst_cap);