Fix TX wait thread stack overflow on Xtensa (boot freeze)

TX_WAIT_THREAD_STACK_SIZE was 1024 bytes, shared by every radio
adapter. On Xtensa (all ESP32/S3 boards) the windowed ABI needs ~3x
the stack of Cortex-M for the same code — Zephyr's own Kconfig
defaults reflect this (IDLE_STACK_SIZE: 1024 if XTENSA vs 320 ARM).

Measured peak usage on Heltec V4 (immediate-mode logging, full TX
cycle including startReceive/lora_config): 1252 bytes — already past
the old 1024 limit. The overflow corrupts the exception frame and
parks CPU0 in _DoubleExceptionVector (LoadProhibited @0x2c), a silent
total freeze at boot. Confirmed via the S3 built-in USB-JTAG:
current thread at crash = lora_tx_wait.

2048 gives ~39% headroom over the measured worst case.

zephcore/adapters/radio/radio_common.h

@@ -24,7 +24,7 @@
 #define RX_RING_SIZE 8  /* ~2 KB; buffers burst arrivals at SF7/BW500 */
 
 /* --- TX wait thread --- */
-#define TX_WAIT_THREAD_STACK_SIZE 1024
+#define TX_WAIT_THREAD_STACK_SIZE 2048
 #define TX_WAIT_THREAD_PRIORITY   10     /* preemptible, below main thread */
 #define TX_TIMEOUT_MS             5000   /* hard timeout for TX completion signal */