LoRa device type + SX1262 kernel driver (first checkpoint) (#565)

Adds sub-GHz radio support to Tactility as a first-class kernel device type, plus a
driver for the Semtech SX1262. Continues the radio work discussed in #342, brought up to
the current kernel driver model (rather than the deprecated tt::hal layer the earlier
prototype targeted).

Drivers/sx126x-module/README.md

@@ -0,0 +1,69 @@
+# SX126x driver module
+
+Kernel driver for the Semtech SX1262 sub-GHz LoRa and (G)FSK transceiver, built on
+[RadioLib](https://github.com/jgromes/RadioLib). It registers a `lora` device
+(see `<tactility/drivers/lora.h>`) that supports LoRa and FSK for TX/RX and LR-FHSS for TX.
+
+The device is a child of an SPI controller. The chip-select line comes from the parent's
+`cs-gpios` entry matching the node's unit address; the `reg` property must match that
+unit address (checked at device start). The reset, busy and DIO1 lines must be
+SoC GPIOs (the RadioLib HAL drives them directly); the optional enable and antenna-select
+lines can live on any GPIO controller, including IO expanders, and are driven to their
+active level while the device is started.
+
+Example (LilyGO T-Deck Max):
+
+```dts
+spi0 {
+    compatible = "espressif,esp32-spi";
+    cs-gpios = <&gpio0 34 GPIO_FLAG_NONE>, // 0: EPD display
+               <&gpio0 48 GPIO_FLAG_NONE>, // 1: SD card
+               <&gpio0 3 GPIO_FLAG_NONE>;  // 2: LoRa radio (SX1262)
+    /* ... */
+
+    radio@2 {
+        compatible = "semtech,sx1262";
+        reg = <2>;
+        pin-reset = <&gpio0 4 GPIO_FLAG_NONE>;
+        pin-dio1 = <&gpio0 5 GPIO_FLAG_NONE>;
+        pin-busy = <&gpio0 6 GPIO_FLAG_NONE>;
+        pin-enable = <&xl9555 1 GPIO_FLAG_NONE>;
+        pin-antenna-select = <&xl9555 4 GPIO_FLAG_NONE>;
+        tcxo-millivolts = <2400>;
+        dio2-as-rf-switch;
+    };
+};
+```
+
+Notes:
+
+- Starting the device selects the antenna path, powers the module and runs a GPIO-only
+  probe (reset pulse, then wait for the chip to drive BUSY low), so an absent or
+  unpowered module fails at device start. The modem itself is initialized when the
+  radio is enabled via `lora_set_enabled()`, which requires a modulation to be set first.
+- Bus sharing: RadioLib drives the chip-select manually across multiple transfers, so
+  each exchange must be atomic on the bus. It runs inside two nested locks — the kernel
+  SPI controller lock (`spi_controller_lock`, the arbiter other kernel drivers on the
+  host cooperate through) as the outer lock, and ESP-IDF's per-host bus lock
+  (`spi_device_acquire_bus`) as the inner one, which additionally blocks the IDF-managed
+  spi_master devices (displays, SD cards) that don't take the controller lock.
+- TX-done is waited for using the packet's time-on-air plus a fixed margin, so slow
+  configurations (high spreading factor / narrow bandwidth) don't false-time-out.
+- Over-current protection defaults to RadioLib's fail-safe 60 mA, which caps the PA
+  current into a bad or disconnected antenna but also caps output below +22 dBm. A
+  board-aware consumer that knows its antenna and PA can raise it via
+  `LORA_PARAMETER_CURRENT_LIMIT` (up to 140 mA) to reach full output power.
+- Radio activity is logged at INFO level (state changes, modem config, TX/RX events);
+  per-call detail sits at DEBUG, which is compiled out unless the sdkconfig log level
+  is raised.
+
+## Roadmap / not yet implemented
+
+- FSK addressed transmission is not exposed through the lora API yet.
+- Antenna-presence detection. The SX1262 has no hardware antenna-detect pin (unlike, e.g.,
+  the u-blox SARA modules' dedicated ANT_DET ADC circuit), and the T-Deck Max antenna
+  connector is not hot-swap-rated. The intended approach is a software heuristic on the
+  radio thread — watching the RX noise floor / RSSI for the signature of a
+  disconnected/open PA load and disabling TX at the driver layer when detected — so the
+  protection lives below the app/OS. This is unproven and deliberately not built yet;
+  feedback on the method is welcome before implementing it.