The GC1109 FEM needs its VFEM_Ctrl pin held HIGH during deep sleep
to keep the LNA active, enabling proper RX sensitivity for
wake-on-packet. Without this, the LNA is unpowered during sleep
and RX wake sensitivity is degraded by ~17dB.
Release RTC holds in begin() after configuring GPIO registers
(not before) to ensure glitch-free pin transitions on wake.
Trade-off: ~6.5mA additional sleep current for significantly
improved wake-on-packet range.
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