Implement SX126X SPI and chip configuration for PoC

- radio.cpp: Add custom SPI bus initialization for boards with non-default pins (conditional on LORA_PIN_SCLK definition). Pass lora_spi object to Module instantiation for SX1262/LR1110.
- radio.cpp: Add TCXO voltage to SX1262/LR1110 chip_begin() as 8th parameter (using SX126X_DIO3_TCXO_VOLTAGE macro or 0 if undefined).
- radio.cpp: Add post-begin SX126X configuration: setDio2AsRfSwitch(), setCurrentLimit(), setRxBoostedGainMode() (conditional on build macros).
- radio.cpp: Fix radio_tx() const conversion for RadioLib transmit() API compatibility.
- config.h: Add LORA_TX_POWER → LORA_POWER_DBM alias for boards like Heltec V3 that define power via LORA_TX_POWER. Use #ifndef guard to avoid redefinition warnings.
- platformio.ini: Add -Wall -Werror to treat warnings as fatal in our code (excluding SDK/framework).
- platformio.ini: Add -std=c99 build flag for KISS C99 support.
- src/kiss.c → src/kiss.cpp: Rename to .cpp so Arduino framework compiles it.

Verified: Builds cleanly for seeed_xiao_s3_wio_sx1262 and heltec_v3 with no warnings.

hardware/seeed/xiao_s3_wio_sx1262/platformio.ini

@@ -1,7 +1,7 @@
 ; Seeed Xiao S3 + Wio SX1262 — ESP32-S3, SX1262
 [env:seeed_xiao_s3_wio_sx1262]
 extends = soc_esp32s3, env:base
-board = seeed_xiao_s3
+board = seeed_xiao_esp32s3
 build_flags =
     ${soc_esp32s3.build_flags}
     ${env:base.build_flags}

platformio.ini

@@ -4,8 +4,8 @@
 
 [platformio]
 default_envs =
-    heltec_v3,
-    rak_rak4631,
+    heltec_v3
+    rak_rak4631
     heltec_v2
 
 extra_configs =
@@ -45,6 +45,10 @@ lib_deps =
 monitor_speed = 115200
 
 build_flags =
+    ; Treat warnings as errors in our code, but not in third-party
+    -Wall -Werror -Wno-error=unused-function
+    ; C99 for KISS implementation
+    -std=c99
     ; Default LoRa radio parameters — override per-board as needed
     -DLORA_FREQ_KHZ=869525UL
     -DLORA_BW_HZ=125000UL

src/config.h

@@ -14,7 +14,11 @@
 #  define LORA_CR 5   /* denominator: coding rate = 4/CR */
 #endif
 #ifndef LORA_POWER_DBM
-#  define LORA_POWER_DBM 14
+#  ifdef LORA_TX_POWER
+#    define LORA_POWER_DBM LORA_TX_POWER
+#  else
+#    define LORA_POWER_DBM 14
+#  endif
 #endif
 #ifndef LORA_SYNCWORD
 #  define LORA_SYNCWORD 0x34   /* LoRa public */

src/radio.cpp

@@ -3,12 +3,27 @@
 #include <RadioLib.h>
 
 /* ── Chip instantiation ─────────────────────────────────────────────────── */
+/* Custom SPI bus — used when board defines non-default SPI pins */
+#if defined(LORA_PIN_SCLK)
+static SPIClass lora_spi(FSPI);
+#endif
+
 #if defined(LORA_CHIP_SX1262)
+#  if defined(LORA_PIN_SCLK)
+static SX1262 radio(new Module(LORA_PIN_NSS, LORA_PIN_DIO1, LORA_PIN_RESET,
+                               LORA_PIN_BUSY, lora_spi));
+#  else
 static SX1262 radio(new Module(LORA_PIN_NSS, LORA_PIN_DIO1, LORA_PIN_RESET,
                                LORA_PIN_BUSY));
+#  endif
 #elif defined(LORA_CHIP_LR1110)
+#  if defined(LORA_PIN_SCLK)
+static LR1110 radio(new Module(LORA_PIN_NSS, LORA_PIN_DIO1, LORA_PIN_RESET,
+                               LORA_PIN_BUSY, lora_spi));
+#  else
 static LR1110 radio(new Module(LORA_PIN_NSS, LORA_PIN_DIO1, LORA_PIN_RESET,
                                LORA_PIN_BUSY));
+#  endif
 #elif defined(LORA_CHIP_SX1276)
 static SX1276 radio(new Module(LORA_PIN_NSS, LORA_PIN_DIO0, LORA_PIN_RESET,
                                RADIOLIB_NC));
@@ -23,13 +38,23 @@ static radio_config_t current_cfg;
 
 #if defined(LORA_CHIP_SX1262)
 static int16_t chip_begin(SX1262 &r, const radio_config_t &cfg) {
+#ifdef SX126X_DIO3_TCXO_VOLTAGE
+    constexpr float tcxo = SX126X_DIO3_TCXO_VOLTAGE;
+#else
+    constexpr float tcxo = 0.0f;
+#endif
     return r.begin(cfg.freq_khz / 1000.0f, cfg.bw_hz / 1000.0f, cfg.sf,
-                   cfg.cr, cfg.syncword, cfg.power_dbm);
+                   cfg.cr, cfg.syncword, cfg.power_dbm, 8, tcxo);
 }
 #elif defined(LORA_CHIP_LR1110)
 static int16_t chip_begin(LR1110 &r, const radio_config_t &cfg) {
+#ifdef SX126X_DIO3_TCXO_VOLTAGE
+    constexpr float tcxo = SX126X_DIO3_TCXO_VOLTAGE;
+#else
+    constexpr float tcxo = 0.0f;
+#endif
     return r.begin(cfg.freq_khz / 1000.0f, cfg.bw_hz / 1000.0f, cfg.sf,
-                   cfg.cr, cfg.syncword, cfg.power_dbm);
+                   cfg.cr, cfg.syncword, cfg.power_dbm, 8, tcxo);
 }
 #elif defined(LORA_CHIP_SX1276)
 static int16_t chip_begin(SX1276 &r, const radio_config_t &cfg) {
@@ -41,6 +66,11 @@ static int16_t chip_begin(SX1276 &r, const radio_config_t &cfg) {
 /* ── C API implementation ────────────────────────────────────────────────── */
 
 int radio_init(void) {
+    /* Initialize custom SPI bus if board uses non-default SPI pins */
+#if defined(LORA_PIN_SCLK)
+    lora_spi.begin(LORA_PIN_SCLK, LORA_PIN_MISO, LORA_PIN_MOSI, LORA_PIN_NSS);
+#endif
+
     current_cfg = {
         .freq_khz = LORA_FREQ_KHZ,
         .bw_hz = LORA_BW_HZ,
@@ -50,11 +80,27 @@ int radio_init(void) {
         .syncword = LORA_SYNCWORD,
     };
     int16_t err = chip_begin(radio, current_cfg);
-    return (err == RADIOLIB_ERR_NONE) ? 0 : (int)err;
+    if (err != RADIOLIB_ERR_NONE)
+        return (int)err;
+
+    /* SX126X post-begin configuration */
+#if defined(LORA_CHIP_SX1262) || defined(LORA_CHIP_LR1110)
+#  ifdef SX126X_DIO2_AS_RF_SWITCH
+    radio.setDio2AsRfSwitch(true);
+#  endif
+#  ifdef SX126X_CURRENT_LIMIT
+    radio.setCurrentLimit(SX126X_CURRENT_LIMIT);
+#  endif
+#  ifdef SX126X_RX_BOOSTED_GAIN
+    radio.setRxBoostedGainMode(true);
+#  endif
+#endif
+
+    return 0;
 }
 
 int radio_tx(const uint8_t *data, size_t len) {
-    int16_t err = radio.transmit(data, len);
+    int16_t err = radio.transmit((uint8_t *)data, len);
     if (err == RADIOLIB_ERR_NONE) {
         radio.startReceive();
     }