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.

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