fix(station-g3): apply FEM PA level at TX start

PA PL1 re-targets the PA's DC-DC supply rail rather than selecting a
logic-level gain, and the serial CLI is serviced on every main-loop pass
regardless of whether a transmit is in flight. A `set radio.fem.txgain`
write could therefore move the rail mid-transmit, while the SX1262 was
still driving the PA at full input power.

Record the requested level in setPAGainEnable() and drive the pin from
setTxModeEnable(), which runs from onBeforeTransmit() ahead of
startTransmit(). The level only matters while transmitting, so deferring
costs nothing.

Document that the pref is saved immediately but applied at the next
transmit, so `get radio.fem.txgain` can lead the hardware until then.

docs/cli_commands.md

@@ -303,6 +303,7 @@ This document provides an overview of CLI commands that can be sent to MeshCore
 - This controls a software-selectable external LoRa FEM transmit gain where the board supports it.
 - On Station G3, remove the PA PL1 jumper to allow software control. `on` selects PA PL1 high/short and `off` selects PA PL1 low/open. The PA PL2 hardware jumper determines whether this switches between power levels 1/3 or 2/4.
 - Select an operating level and SX1262 transmit power that comply with local RF limits and the Station G3 power-supply requirements.
+- The setting is saved immediately, but on Station G3 the level is applied to the hardware at the start of the next transmit, so that the PA supply rail is never re-targeted while the PA is being driven. `get` reports the configured state, which may lead the hardware until the node next transmits.
 
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