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>

LICENSE

@@ -0,0 +1,27 @@
+MIT License
+
+Copyright (c) 2026 Scott Penrose / Digital Dimensions
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+---
+
+The companion protocol implemented here was derived from the MeshCore Companion
+Radio protocol as documented in the MeshCore project and the reference client
+meshcore.js (MIT, Copyright (c) Liam Cottle, https://github.com/meshcore-dev/meshcore.js).