editor: clangd and cmake work from a committed vscode workspace

The three files libavr's consumers carry: .vscode/settings.json names the
atmega328p-generated database and passes --query-driver, .clangd holds the
stand-ins clang needs for GCC's AVR dialect -- and no database, because this
driver is made to be vendored and the file travels with it -- and
extensions.json names the two extensions. The libavr pin advances to the
editor-audit fixes, without which every TU inherits device.hpp's
alias-shadowing errors. The preset builds green from the pin and clangd
reports zero errors on the example TU.

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

.clangd

@@ -0,0 +1,32 @@
+# Editor accommodations for the second frontend. No compilation database is
+# named here: this driver is made to be vendored, and this file travels with
+# it — a consumer's own database then covers these headers, at the consumer's
+# chip. 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:
+    # 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