fix(radio): disable Pwr save (RX duty-cycle) — unsafe across mixed firmware

Field report: a user on the stock SF8 "EU/UK (Narrow)" preset saw reception
drop from ~1-5 msg/min to ~1/3h with Pwr save on, unaffected by a better
antenna. Root cause, confirmed against the SX1262 datasheet and RadioLib's
own maintainers (jgromes/RadioLib#1597, closed as inherent chip behaviour):
the SX126x's duty-cycle preamble detection needs the sender's actual
preamble to closely match what we've configured our receiver to expect.
A mismatch — e.g. a repeater still on pre-v1.16 firmware sending 16 symbols
against our 32 — doesn't cost a little sensitivity, it silently drops every
packet from that sender regardless of signal strength. There's no local
parameter fix: shortening minSymbols to tolerate shorter preambles directly
shortens the wake-window's correlator dwell time, trading the preamble
mismatch failure mode for a marginal-signal one instead. Checked whether
IoTThinks' MeshCore fork solved this differently — it doesn't hit the
problem at all, because its "power saving" only light-sleeps the MCU on the
radio's own GPIO interrupt and never touches RX duty-cycle.

examples/companion_radio/MyMesh.h now defines FEAT_RX_POWERSAVE 0, gating
out the Settings row, the Diagnostics RXPS watchdog row, and every call site
that would apply rx_powersave to the radio or CAD auto-enable — including
forcing setPowerSaving(false) unconditionally so a stale rx_powersave=1 byte
from before this change can't do anything either. The real duty-cycle
implementation in RadioLibWrapper/CustomSX1262Wrapper is left in place,
unneutered, for if a network-wide compatibility mechanism (e.g. the
still-unused ADV_FEAT1/FEAT2 advert bits) is ever built to make it safe.

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

examples/companion_radio/MyMesh.cpp

@@ -287,7 +287,14 @@ bool MyMesh::getCADEnabled() const {
   // _prefs.cad_enabled itself has no UI/CLI exposure yet on companion_radio
   // (unlike simple_repeater's CommonCLI `cad` command) — it's wired and
   // persisted for a future manual override, but always 0 today.
+  // rx_powersave is never actually applied to the radio while FEAT_RX_POWERSAVE
+  // is 0 (see MyMesh.h) -- don't let a stale persisted byte from before that
+  // still auto-enable CAD here for a duty-cycle mode that isn't running.
+#if FEAT_RX_POWERSAVE
   return _prefs.cad_enabled || (_prefs.rx_powersave && !_prefs.client_repeat);
+#else
+  return _prefs.cad_enabled;
+#endif
 }
 
 int MyMesh::calcRxDelay(float score, uint32_t air_time) const {
@@ -1991,7 +1998,11 @@ void MyMesh::begin(bool has_display) {
   applyRepeaterRadio();   // companion params, or the repeater profile if relaying with one set
   applyApc();                                         // sets TX power to the ceiling and arms APC if enabled
   radio_driver.setRxBoostedGainMode(_prefs.rx_boosted_gain);
+#if FEAT_RX_POWERSAVE
   radio_driver.setPowerSaving(_prefs.rx_powersave && !_prefs.client_repeat);   // duty-cycle RX off while repeating (must hear all traffic)
+#else
+  radio_driver.setPowerSaving(false);   // see MyMesh.h FEAT_RX_POWERSAVE -- ignore any stale persisted rx_powersave byte
+#endif
   board.setLoRaFemLnaEnabled(_prefs.radio_fem_rxgain);
   board.setLoRaFemPaGainEnabled(_prefs.radio_fem_txgain);
   MESH_DEBUG_PRINTLN("RX Boosted Gain Mode: %s",
@@ -2482,7 +2493,11 @@ void MyMesh::handleCmdFrame(size_t len) {
       // Keep the "repeating ⇒ continuous RX, full TX power" invariants when repeat
       // is toggled via the app, mirroring the on-device path (a repeater must hear
       // all traffic and relay at consistent power).
+#if FEAT_RX_POWERSAVE
       radio_driver.setPowerSaving(_prefs.rx_powersave && !_prefs.client_repeat);
+#else
+      radio_driver.setPowerSaving(false);   // see MyMesh.h FEAT_RX_POWERSAVE -- ignore any stale persisted rx_powersave byte
+#endif
       applyApc();   // pins power to the ceiling; apcActive() keeps it there while repeating
       MESH_DEBUG_PRINTLN("OK: CMD_SET_RADIO_PARAMS: f=%d, bw=%d, sf=%d, cr=%d", freq, bw, (uint32_t)sf,
                          (uint32_t)cr);