Restructure into dual-purpose meshcore_c library

Remove stale byte-identical root duplicates and promote the canonical
library to the repo root: one source of truth (src/meshcore_companion.{c,h})
serving both a portable C library and a publishable C++ Arduino/PlatformIO
library.

- Portable C99 core + C++ Arduino wrapper in src/
- Arduino sketch in examples/, new Linux tty example in examples-linux/
- CMakeLists.txt for the Linux/native host build (core + example + test)
- Host codec unit test in test/
- README rewritten around the two purposes

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

library.properties

@@ -0,0 +1,9 @@
+name=MeshCoreCompanion
+version=0.1.0
+author=Scott Penrose
+maintainer=Scott Penrose
+sentence=Client for the MeshCore Companion Radio serial protocol.
+paragraph=Connect a host MCU (e.g. an ESP32 display board) to a separate MeshCore companion radio over UART/USB serial. Portable C99 protocol core plus a thin Arduino wrapper. Send/receive on channels, set channel PSKs, read SNR/path metadata. Auto-drains the radio message queue.
+category=Communication
+url=https://github.com/digitaldimensions/MeshCoreCompanion
+architectures=esp32,nrf52,stm32,rp2040