fix LNA/FEM control

docs/Repeater_CLI_commands.md

@@ -260,7 +260,7 @@ All `set uplink.*` changes are saved immediately and only applied after reboot.
 | `get path.hash.mode` | Path hashing algorithm: `0`, `1`, or `2` |
 | `get loop.detect` | Loop detection level: `off`, `minimal`, `moderate`, or `strict` |
 | `get radio.rxgain` | RX gain boost: `on` or `off` |
-| `get radio.fem.rxgain` | External FEM/LNA active during RX: `on` or `off` (default `on`) |
+| `get radio.fem.rxgain` | External FEM's LNA in the RX path: `on` (through the LNA) or `off` (bypassed). Default `on` |
 | `get rxduty` | RX duty cycle mode: `0` or `1` |
 | `get gps duty` | Now-effective GPS duty interval in seconds (`always on (0)` when continuous) |
 | `get gps diag` | What the last GPS module-configuration attempt did — which path ran, bytes sent, and tracked satellites per constellation. See **GPS configuration diagnostics** in the GPS section for the field reference |
@@ -312,7 +312,7 @@ Changes are persisted immediately unless noted. Some require a reboot.
 | `set path.hash.mode <mode>` | 0, 1, or 2 | Path hashing algorithm |
 | `set loop.detect <mode>` | `off`, `minimal`, `moderate`, `strict` | Loop detection sensitivity |
 | `set radio.rxgain <0\|1\|on\|off>` | | RX gain boost, applied live. Replies `Error: unsupported` on radios without RX boost (SX127x); the pref is still saved. |
-| `set radio.fem.rxgain <0\|1\|on\|off>` | default **1** | External FEM/LNA gating in the RX direction, applied live. Supported only where the FEM's chip-enable is wired to the radio node as `antenna-enable-gpios` — today `heltec_t096`, `heltec_wireless_tracker_v2`, `heltec_wifi_lora32_v4` and `heltec_wifi_lora32_v43`. Every other board reports `Error: unsupported`, for one of three reasons: it has no external amplifier at all; its amplifier has no separate receive-side enable, only the DIO2/TXEN/RXEN transmit-receive switch (`station_g2`, `gat562_30s`, `ikoka_nano_30dbm`, `promicro_sx1262`); or its amplifier enable is a standalone always-on regulator outside the radio node (`rak3401_1watt` — moving that pin onto the lora node as `antenna-enable-gpios` is what would enable it). Non-SX126x radios (LR1110, LR2021, SX127x) never support it. Setting `0` drops the FEM's RX gain (~16 dB on the SKY66122) and its supply current (~6.5 mA) — sensitivity for battery life, not a tuning knob. Transmit always uses the FEM, and the driver's idle/sleep gating is unchanged. Replies `Error: unsupported` on non-SX126x radios and on boards with no FEM wired; the pref is still saved. |
+| `set radio.fem.rxgain <0\|1\|on\|off>` | default **1** | Routes receive through the external FEM's LNA (`1`) or around it via the FEM's bypass path (`0`), applied live. Sensitivity for battery life — `0` costs roughly 17 dB and saves the LNA's supply current. Transmit, and the driver's idle/sleep gating of the FEM, are unaffected either way. Supported only where the FEM's receive path is software-selectable and that select line is wired to the radio node as `lna-bypass-gpios` — today the three KCT8103L boards, `heltec_t096`, `heltec_wireless_tracker_v2` and `heltec_wifi_lora32_v43`. Every other board reports `Error: unsupported`: `heltec_wifi_lora32_v4`'s GC1109 has no receive-path select (its CPS is don't-care in RX, same as MeshCore); `station_g2`, `gat562_30s`, `ikoka_nano_30dbm` and `promicro_sx1262` have only the DIO2/TXEN/RXEN transmit-receive switch; `rak3401_1watt`'s SKY66122 is enabled by a standalone always-on regulator outside the radio node; and non-SX126x radios (LR1110, LR2021, SX127x) never implement it. The pref is still saved when unsupported. **Do not expect the FEM's chip-enable to be the knob** — deasserting `antenna-enable-gpios` in RX shuts the part down and takes the through path with it (~69 dB measured on a V4.3), which is what 1.17.2 did before this moved to `lna-bypass-gpios`. |
 | `set rxduty <0\|1\|on\|off>` | | RX duty cycle mode *(reboot required)*. Window timing auto-sized per SF/BW/preamble from the SX126x datasheet constraints (boot log line `rxduty:` shows the result). Zero-loss guarantee assumes senders on preamble-32 firmware (current MeshCore at SF≤8); legacy preamble-16 senders are only caught ~50% worst-phase — keep off until the local mesh has converted. Presets with 16-symbol preambles (SF≥9) fall back to continuous RX automatically. |
 | `set adc.multiplier <mult>` | `0` (use board default) or 100–30000 | Battery voltage ADC calibration multiplier, set directly. Rejects non-numeric input, NaN/inf and negatives. |
 | `set adc.multiplier target <mv>` | 3000–4400 mV | Calibrate against a voltage you measured with a multimeter: rescales the current multiplier so the ADC reads `<mv>`. Replies with the old and new multiplier plus the before/after reading. `Error: no ADC reading on this board` if the board has no battery ADC. |