editor: clangd and cmake work from a committed vscode workspace

The three files libavr's consumers carry, in the leaf-application shape:
nothing vendors this repo, so .clangd names the one build tree an editor
should read (build/atmega328p-generated) beside the stand-ins clang needs
for GCC's AVR dialect; .vscode/settings.json passes --query-driver and names
the toolchain prefix for the window that opens this folder. The libavr pin
advances to the editor-audit fixes. One residue stays red and is the
documented frontend divergence, not a defect: thermistor.hpp's consteval
Beta-equation table folds __builtin_log under GCC, which clang refuses to
fold in a constant expression -- the same class as libavr's breathe example
and its __builtin_cos.

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

.vscode/extensions.json

@@ -0,0 +1,6 @@
+{
+	"recommendations": [
+		"llvm-vs-code-extensions.vscode-clangd",
+		"ms-vscode.cmake-tools"
+	]
+}