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>

CMakeLists.txt

@@ -0,0 +1,34 @@
+# CMakeLists.txt -- Linux / native (host) build for the portable C core.
+#
+# This builds the dependency-free C99 core (src/meshcore_companion.c), the host
+# unit test, and the Linux tty example. It deliberately does NOT build the C++
+# Arduino wrapper (src/MeshCoreCompanion.{h,cpp}) -- that requires <Arduino.h>
+# and is compiled by the Arduino IDE / PlatformIO instead. The Arduino manifests
+# (library.properties, library.json) at the repo root are simply ignored here.
+#
+#   cmake -B build && cmake --build build
+#   ctest --test-dir build --output-on-failure
+#
+# SPDX-License-Identifier: MIT
+
+cmake_minimum_required(VERSION 3.13)
+project(meshcore_c LANGUAGES C)
+
+set(CMAKE_C_STANDARD 99)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+
+add_compile_options(-Wall -Wextra)
+
+# Portable protocol core, reusable on any platform with a byte transport.
+add_library(meshcore_companion STATIC src/meshcore_companion.c)
+target_include_directories(meshcore_companion PUBLIC src)
+
+# Linux example: drives a companion radio over any tty (USB-CDC or raw UART).
+add_executable(meshcore_tty examples-linux/tty_bridge/meshcore_tty.c)
+target_link_libraries(meshcore_tty PRIVATE meshcore_companion)
+
+# Host unit test for the codec (no hardware required).
+enable_testing()
+add_executable(test_codec test/test_codec.c)
+target_link_libraries(test_codec PRIVATE meshcore_companion)
+add_test(NAME codec COMMAND test_codec)