feat(power): battery saving — hardware duty-cycle RX + adaptive TX power

Two independent, default-off toggles under Settings › Radio.

Pwr save: hardware RX duty-cycle (SX126x SetRxDutyCycle via
startReceiveDutyCycleAuto). The chip cycles RX↔sleep and wakes on a preamble —
no MCU state machine; recvRaw reads the packet exactly as in continuous RX.
Falls back to continuous RX on non-SX126x. (Replaces an earlier software-CAD
state machine that fought the hardware: polling a warm-sleeping chip gave a
phantom-busy channel that stalled TX ~4 s and dropped ACKs in the scan gaps.)

Auto pwr: Adaptive Power Control. tx_power_dbm becomes a ceiling; actual TX
power tracks the reverse-link SNR margin (measured above the per-SF demod floor,
EWMA-smoothed, proportional step with a deadband). Feedback comes from direct /
room-server ACKs and, for channels (no ACK), from hearing a repeater rebroadcast
our own flood; a lost confirmation ramps power back up so channel sends can't get
stranded below what the repeaters can hear.

Prefs schema 0xC0DE0009 (rx_powersave, tx_apc). Radio page / name bar show the
live TX power; noise floor reads n/a while duty-cycling.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -44,6 +44,10 @@ Solo firmware thread: https://discord.com/channels/1495203904898728149/150529433
 
 - [Tools Screen](./docs/solo_features/tools_screen/tools_screen.md) — GPS trail & waypoints, compass, nearby nodes (with ping & navigate), ringtone editor, auto-reply bot, auto-advert
 
+- **Battery saving (radio)** — two optional, independent toggles under Settings › Radio:
+  - **Pwr save** — hardware duty-cycle receive (SX126x `SetRxDutyCycle`): the radio cycles RX↔sleep on its own and wakes on a preamble, cutting average RX current with only a little added receive latency
+  - **Auto pwr** — Adaptive Power Control: trims actual TX power on strong links (from ACK SNR) and ramps back up to the configured ceiling on weak/lost links; the home screen shows the live power
+
 ### E-ink Display
 
 The e-ink variant targets the Wio Tracker L1 fitted with a 2.13″ GxEPD2 panel (250 × 122 px). All screens have been adapted for the e-ink panel: