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>

.clangd

@@ -0,0 +1,30 @@
+CompileFlags:
+  # Named here rather than in the editor's settings because nothing vendors
+  # this repo: the one build tree is the one an editor should read.
+  CompilationDatabase: build/atmega328p-generated
+  Add:
+    # clang has no 24-bit integer and GCC's are keywords, not macros, so the
+    # editor needs a stand-in for avr::uint24_t. The next width up is the only
+    # one available — clang rejects _BitInt(24) on this target.
+    - -D__uint24=unsigned long
+    - -D__int24=long
+    # clangd forwards the driver's system includes but not its own header
+    # directory, so <stdint.h> resolves to avr-libc's, which still gates the
+    # limit and constant macros on the C++98 opt-in.
+    - -D__STDC_LIMIT_MACROS
+    - -D__STDC_CONSTANT_MACROS
+    # isr::emit spells a vector number into [[gnu::signal(N)]], which clang
+    # rejects rather than ignores — enough of them in one TU to reach the
+    # default limit of 19 inside the headers and truncate the parse.
+    - -ferror-limit=0
+  Remove:
+    # The build promotes warnings for the compiler that has to be right about
+    # them; in the editor the flag paints a second frontend's opinions in the
+    # colour reserved for things that do not compile.
+    - -Werror
+Diagnostics:
+  Suppress:
+    # clang's AVR `signal` attribute takes no arguments and it knows none of
+    # progmem, naked or OS_main. A misspelling is what the build is for.
+    - attribute_wrong_number_arguments
+    - unknown-attributes

.vscode/extensions.json

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

.vscode/settings.json

@@ -0,0 +1,34 @@
+{
+	// clangd is the language server; the cpptools engine would parse every file
+	// a second time and disagree, since nothing tells it about a cross
+	// compiler. Which database to read is in .clangd.
+	"C_Cpp.intelliSenseEngine": "disabled",
+
+	// --query-driver lets clangd ask the cross compiler for its own system
+	// includes and target; without it every standard header is missing.
+	"clangd.arguments": [
+		"--query-driver=**avr-g++*",
+		"--header-insertion=never"
+	],
+
+	// The presets are the build interface; the toolchain file inside the
+	// libavr submodule is the one place the compiler is chosen. The submodule
+	// carries no local/toolchain for it to discover, so the prefix is named
+	// here, for the window that opens this folder.
+	"cmake.useCMakePresets": "always",
+	"cmake.environment": {
+		"LIBAVR_TOOLCHAIN": "D:/dev/libavr/local/toolchain/avr-gcc-16.1.0-mingw"
+	},
+	"cmake.configureOnOpen": true,
+	"cmake.options.statusBarVisibility": "compact",
+
+	"files.watcherExclude": {
+		"**/build/**": true,
+		"**/libavr/**": true
+	},
+
+	"files.associations": {
+		".clangd": "yaml",
+		".clang-format": "yaml"
+	}
+}