build: libavr rides as the pinned submodule

The submodule replaces FetchContent and the unpinned forge fallback;
LIBAVR_ROOT stays as the tandem-development override, the presets take the
toolchain file from the submodule, and the Studio project's include path
anchors there — correct by construction.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CMakePresets.json

@@ -6,7 +6,7 @@
 			"hidden": true,
 			"generator": "Ninja",
 			"binaryDir": "${sourceDir}/build/${presetName}",
-			"toolchainFile": "$env{LIBAVR_ROOT}/cmake/avr-toolchain.cmake",
+			"toolchainFile": "${sourceDir}/libavr/cmake/avr-toolchain.cmake",
 			"cacheVariables": {
 				"CMAKE_BUILD_TYPE": "Release",
 				"CMAKE_EXPORT_COMPILE_COMMANDS": "ON",