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>

.clangd

@@ -0,0 +1,37 @@
+# Editor accommodations for the second frontend. No compilation database is
+# named here: this repo rides as a submodule in its consumers, and this file
+# travels with it — a consumer's own database then covers these sources, with
+# that project's loader flags. The checkout that is opened as a folder names
+# its build tree in .vscode/settings.json instead.
+CompileFlags:
+  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:
+    # Codegen shaping the loader TUs carry and clang has no spelling for.
+    - -fira-algorithm=*
+    - -fno-split-wide-types
+    - -fno-tree-ter
+    - -fno-ivopts
+    # 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 +1,37 @@
+{
+	// 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.
+	"C_Cpp.intelliSenseEngine": "disabled",
+
+	// --query-driver lets clangd ask the cross compiler for its own system
+	// includes and target. The database is named here rather than in .clangd
+	// because that file travels with the driver into a consumer's submodule,
+	// where a build tree of this repo's own need not exist.
+	"clangd.arguments": [
+		"--compile-commands-dir=${workspaceFolder}/build/atmega328p-generated",
+		"--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"
+	}
+}