editor: clangd and cmake work from a committed vscode workspace

The three files libavr's consumers carry, in the vendored shape: this repo
rides as a submodule (wateralarm), so .clangd names no database -- the
checkout that is opened as a folder names build/atmega328p-generated in
.vscode/settings.json -- and holds the stand-ins clang needs for GCC's AVR
dialect plus the removals for the codegen flags the loader TUs carry and
clang has no spelling for (-fira-algorithm, -fno-split-wide-types,
-fno-tree-ter, -fno-ivopts). The libavr pin advances to the editor-audit
fixes. The preset builds green from the pin and clangd reports zero errors
on pureboot.cpp.

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"
+	]
+}