feat(boards): LilyGo T-Beam (classic) support

First classic-ESP32 (non-S3) board in the firmware, and first SX127x
radio. ESP32 + SX1276 + SSD1306 OLED + AXP192 PMIC, CP2102 USB-UART
bridge. Pin map verified from MeshCore variants/lilygo_tbeam_SX1276.

The AXP192 gates power to the LoRa chip and OLED. Its I²C bus is
shared with the display, so the firmware builds I²C0 in blocking mode
first for the PMIC init, then converts the same handle to async and
hands it to the display task — avoids double-consuming the peripheral.

Lora-phy's Sx127x support drives DIO0 as the sole IRQ (SX127x has no
BUSY line); the chip runs from a crystal, tx_boost routes +20 dBm
through PA_BOOST.

The `esp-hal` / `esp-rtos` / `esp-backtrace` / `esp-println` deps have
been restructured so the chip flavour (esp32 vs esp32s3) is selected
at the per-board feature level instead of baked into the dep lines.
This keeps the existing S3 boards building on esp32s3 while the new
T-Beam feature selects classic esp32 via the same dependency set.

LoRaBoard gains supported_sf_bitmap() / supported_bw_bitmap() methods
(with SX126x defaults) so the T-Beam can report SF6–SF12 via GET_INFO
— the SX1276 has no SF5. radio_chip_id() reports 0x0011 per
donglora-protocol RadioChipId::Sx1276. freq_range_hz widens to the
SX1276 datasheet span of 137 MHz–1020 MHz.

New modules:
- src/hal/esp32.rs           — classic-ESP32 MCU primitives
- src/board/esp32.rs         — Sx127x radio + SSD1306 display pipe
- src/board/lilygo_tbeam.rs  — the board file
- src/driver/axp.rs          — AXP192 init added alongside AXP2101

Cargo.toml

@@ -45,22 +45,20 @@ ssd1306 = { version = "0.10", features = ["async"] }
 embedded-io-async = "0.7"
 embassy-futures = "0.1"
 
-# ESP32-S3 (Heltec V3) — requires esp toolchain (espup)
+# Xtensa ESP crates — requires esp toolchain (espup). The chip feature
+# (esp32 / esp32s3) is selected per board via the board-level feature flag
+# below, so the same dep lines support both classic ESP32 and ESP32-S3.
 esp-backtrace = { version = "0.18", optional = true, features = [
-  "esp32s3",
   "panic-handler",
   "defmt",
 ] }
 esp-hal = { version = "1.0", optional = true, features = [
-  "esp32s3",
   "unstable",
 ] }
 esp-println = { version = "0.16", optional = true, features = [
-  "esp32s3",
   "defmt-espflash",
 ] }
 esp-rtos = { version = "0.2", optional = true, features = [
-  "esp32s3",
   "embassy",
 ] }
 
@@ -110,6 +108,10 @@ heltec_v3 = [
   "dep:esp-hal",
   "dep:esp-println",
   "dep:esp-rtos",
+  "esp-backtrace/esp32s3",
+  "esp-hal/esp32s3",
+  "esp-println/esp32s3",
+  "esp-rtos/esp32s3",
 ]
 heltec_v3_uart = [
   "embassy",
@@ -117,6 +119,10 @@ heltec_v3_uart = [
   "dep:esp-hal",
   "dep:esp-println",
   "dep:esp-rtos",
+  "esp-backtrace/esp32s3",
+  "esp-hal/esp32s3",
+  "esp-println/esp32s3",
+  "esp-rtos/esp32s3",
 ]
 heltec_v4 = [
   "embassy",
@@ -124,6 +130,10 @@ heltec_v4 = [
   "dep:esp-hal",
   "dep:esp-println",
   "dep:esp-rtos",
+  "esp-backtrace/esp32s3",
+  "esp-hal/esp32s3",
+  "esp-println/esp32s3",
+  "esp-rtos/esp32s3",
 ]
 elecrow_thinknode_m2 = [
   "embassy",
@@ -131,6 +141,10 @@ elecrow_thinknode_m2 = [
   "dep:esp-hal",
   "dep:esp-println",
   "dep:esp-rtos",
+  "esp-backtrace/esp32s3",
+  "esp-hal/esp32s3",
+  "esp-println/esp32s3",
+  "esp-rtos/esp32s3",
 ]
 lilygo_tbeam_supreme = [
   "embassy",
@@ -138,6 +152,21 @@ lilygo_tbeam_supreme = [
   "dep:esp-hal",
   "dep:esp-println",
   "dep:esp-rtos",
+  "esp-backtrace/esp32s3",
+  "esp-hal/esp32s3",
+  "esp-println/esp32s3",
+  "esp-rtos/esp32s3",
+]
+lilygo_tbeam = [
+  "embassy",
+  "dep:esp-backtrace",
+  "dep:esp-hal",
+  "dep:esp-println",
+  "dep:esp-rtos",
+  "esp-backtrace/esp32",
+  "esp-hal/esp32",
+  "esp-println/esp32",
+  "esp-rtos/esp32",
 ]
 rak_wisblock_4631 = [
   "embassy",