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

README.md

@@ -24,6 +24,7 @@ All commands are run from the `firmware/` directory.
 | Heltec V3 (UART) | `heltec_v3_uart` | ESP32-S3 | SX1262 | CP2102 bridge | xtensa-esp32s3-none-elf |
 | Heltec V4 | `heltec_v4` | ESP32-S3 | SX1262 | Native CDC-ACM | xtensa-esp32s3-none-elf |
 | ELECROW ThinkNode-M2 | `elecrow_thinknode_m2` | ESP32-S3 | SX1262 | CP2102 bridge | xtensa-esp32s3-none-elf |
+| LilyGo T-Beam | `lilygo_tbeam` | ESP32 | SX1276 | CP2102 bridge | xtensa-esp32-none-elf |
 | LilyGo T-Beam S3 Supreme | `lilygo_tbeam_supreme` | ESP32-S3 | SX1262 | Native CDC-ACM | xtensa-esp32s3-none-elf |
 | RAK WisBlock 4631 | `rak_wisblock_4631` | nRF52840 | SX1262 | Native CDC-ACM | thumbv7em-none-eabihf |
 | Waveshare RP2040-LoRa | `waveshare_rp2040_lora` | RP2040 | SX1262 | Native CDC-ACM | thumbv6m-none-eabi |