stm32_link: sync iin_avg_ma comment with live firmware protocol

Firmware now runs BOTH iin limits (IIN_MAX reverse-feed and the IIN_MIN
forward OCP, raised to -47A) on the 8-sample boxcar; wire format is
unchanged (verified field-by-field against the live code64 console,
including a synthetic-frame decode). Note the field is a raw int16 mA
cast and wraps at +-32.768A (observed +26.5A at a true -39.0A) -- the
float iin field is the honest monitor above that.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

testbench/stm32_link.py

@@ -164,7 +164,7 @@ class BroadcastData:
     param_value: int = 0   # raw bits; interpret via decode_param_bits(param_type, ...)
     pong: int = 0          # increments when the MCU processes CMD_PING
     sys_current_ma: int = 0   # Vout-rail housekeeping current, mA
-    iin_avg_ma: int = 0       # 8-sample boxcar of iin, mA (the IIN_MAX trip quantity)
+    iin_avg_ma: int = 0       # 8-sample boxcar of iin, mA (the IIN_MAX/IIN_MIN trip quantity)
     timestamp: float = field(default_factory=time.time)
 
     @property