feat: Projektstruktur nach P1204R0

Layout, Build und Konventionen analog zu mydb und cxx_scaffold_cli:
Header und Quellen nebeneinander in playground/, Unit-Tests als
.test.c/.test.cpp daneben, Integrationstests in tests/.

Geruest-Module counter (C, Unity) und notebook (C++, doctest) zeigen
jede Regel einmal an lauffaehigem Code.

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

.clangd

@@ -0,0 +1,34 @@
+# clangd project configuration.
+#
+# CompilationDatabase points at the build tree directly. Note what this replaces:
+# copying or symlinking compile_commands.json into the project root, which goes
+# stale silently and is how the previous setup broke.
+CompileFlags:
+  CompilationDatabase: build/debug
+
+Index:
+  Background: Build
+  StandardLibrary: Yes
+
+Diagnostics:
+  # misc-include-cleaner is already disabled in .clang-tidy; these are clangd's
+  # own equivalents and would contradict it.
+  UnusedIncludes: None
+  MissingIncludes: None
+
+InlayHints:
+  Enabled: Yes
+  ParameterNames: Yes
+  DeducedTypes: Yes
+
+---
+# Vendored code is not ours: no diagnostics, no clang-tidy, ever. Without this,
+# opening third_party/doctest/doctest.h shows hundreds of findings from our own
+# strict check list against code we will never edit.
+If:
+  PathMatch: .*/third_party/.*
+Diagnostics:
+  Suppress: '*'
+  ClangTidy:
+    Remove:
+      - '*'