Add ESP-IDF and STM32 C examples; document platform examples

Both compile the same portable core (src/meshcore_companion.c) and differ only
in the UART transport:
- examples-esp-idf/tty_bridge: full ESP-IDF project (UART driver), core
  compiled directly via the component CMakeLists (no copy)
- examples-stm32/uart_bridge: HAL drop-in (meshcore_setup/meshcore_poll) for a
  CubeMX/CubeIDE project, with integration README

README updated: new examples in layout + an 'Other platform examples' section.
Verified host build/test still pass and both new examples pass -Wall -Wextra
syntax checks against stubbed platform headers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

examples-esp-idf/tty_bridge/README.md

@@ -0,0 +1,30 @@
+# ESP-IDF example: tty_bridge
+
+Drives a MeshCore Companion Radio over a hardware UART using the ESP-IDF UART
+driver. The protocol logic is the repo's portable C core
+(`src/meshcore_companion.c`) — this example only supplies the transport and an
+`app_main()` loop.
+
+## Wiring
+
+| Companion radio | ESP32 (default pins) |
+|-----------------|----------------------|
+| TX → host RX    | GPIO16 (`MC_UART_RX_PIN`) |
+| RX ← host TX    | GPIO17 (`MC_UART_TX_PIN`) |
+| GND             | GND |
+
+Change the pins/UART at the top of `main/main.c`. The radio must run the serial
+companion firmware (`companion_radio_usb`) bound to this UART.
+
+## Build
+
+Requires an installed [ESP-IDF](https://docs.espressif.com/projects/esp-idf/)
+(v5.x). From this directory:
+
+```sh
+idf.py set-target esp32s3      # or esp32, esp32c3, ...
+idf.py build flash monitor
+```
+
+`main/CMakeLists.txt` compiles `../../../src/meshcore_companion.c` directly, so
+there is no copy of the core to keep in sync.