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>

docs/solo_features/settings_screen/settings_screen.md

@@ -71,8 +71,9 @@ Lists all available home screen pages. For each entry:
 | SF        | 5–12       | LEFT/RIGHT. Spreading factor. |
 | BW        | 7.8–500 kHz | LEFT/RIGHT cycles the standard LoRa bandwidths. |
 | CR        | 5–8        | LEFT/RIGHT. Coding rate (4/5–4/8). |
-| Pwr save  | ON / OFF   | **Battery saver.** Hardware duty-cycle receive (SX126x only): cycles RX↔sleep, wakes on preamble, cuts average RX current at the cost of some latency. **Forced off (`--`) while the repeater is on** — restored once it's switched off. A background watchdog auto-recovers if the sequencer gets stuck (soft re-arm, then a full reset) — see Tools › Diagnostics for the counts. |
 | Auto pwr  | ON / OFF   | **Adaptive Power Control.** Lowers TX power on strong links, ramps back to the **TX Pwr** ceiling on weak/lost ones. Link quality from DM ACK SNR, or — for channels (no ACK) — a repeater's rebroadcast. Live power shown on the radio page/name bar. Default OFF. **Suppressed (`--`) while the repeater is on** — restored once it's switched off. |
+
+There is no "Pwr save" row: hardware RX duty-cycle receive was tried and disabled (`FEAT_RX_POWERSAVE 0` in `examples/companion_radio/MyMesh.h`) — the SX126x's duty-cycle preamble detection needs the sender's actual preamble to exactly match what we configure, which a mixed-firmware mesh can't guarantee (silently drops every packet from a mismatched sender, no matter the signal strength). See `docs/development/roadmap.md` for the full writeup.
 | Scope     | list       | Shows the list's **default** scope. **Enter** opens the **SCOPE** list: `*` (wildcard = unscoped, always first, can't be renamed or deleted) plus up to 8 named scopes of your own (e.g. `pl`). Typing a name derives a shared key the same way on every device, so any device that types the same name lands on the same scope automatically, no key exchange needed. **Enter** on a row opens **Set as default** / **Rename** / **Delete** (delete confirms first); **+ Add scope** at the bottom opens the keyboard. The **default** scope (marked `[default]`) governs **DMs** and the **repeater's own relay slot**; each **channel** carries its own pick — set from the channel's context menu (see [Message Screen](../message_screen/message_screen.md)) — and a channel left on `*` sends unscoped. Scopes tag flood traffic so repeaters can tell your community's messages apart from others sharing the same frequency; paired with **Tools › Repeater › Scope only** it's also what this device relays for in repeater mode. The default also syncs both ways with the phone app's default-scope setting. Not encryption — message content is unaffected either way. Upgrading from a build with the old single Scope field carries it over as the default entry and seeds every existing channel with it, so nothing changes on the air. |
 
 |           OLED            |           E-Ink           |