M3 step 1: RadioLib and lora probe

The probe finds the SX1262 (TCXO 1.8 V works) and measures the antenna
path: the Cap's PI4IOE5V6408 at 0x43 must drive P0 high, or the receiver
is deaf (-111.9 dBm flat vs -87 to -94 dBm with P0 high). DIO2 makes no
difference to reception. Receive only; the radio is left asleep.

RadioLib 7.8.1 + probe: +23.6 KB flash, +656 B static RAM (release).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

platformio.ini

@@ -19,6 +19,7 @@ build_flags =
     -DARDUINO_USB_MODE=1
 lib_deps =
     m5stack/M5Cardputer @ 1.1.1
+    jgromes/RadioLib @ 7.8.1
 test_ignore = *
 ; Smaller TLS buffers (M2): the framework is rebuilt with these settings (pioarduino "hybrid
 ; compile"). Receive stays 16 KB (servers send full TLS records); send drops to 4 KB (IRC lines are