bastion: add freestanding C++ standard library (libcxx)feature/freestanding-libcxx

Header-only implementation of a C++ standard library subset for the
freestanding kernel, using Clang builtins wherever possible. Provides
19 public headers (~35 internal files): type_traits, utility, memory,
algorithm, functional, concepts, array, string_view, span, optional,
expected, variant, tuple, bit, limits, new, initializer_list, cstdint,
cstddef, and source_location.

Integration changes:
- kernel/Makefile: add -Ilib/libcxx/include to include path
- kernel/lib/string.cpp: add memchr and strncmp (needed by string_view)
- kernel/lib/cxxabi.cpp: guard placement new against <new> header conflict
- kernel/include/kernel/kernel.h: Panic() now uses std::source_location
- kernel/core/kernel.cpp: Panic() implementation updated to match

Builds cleanly on both x86_64 and aarch64 with zero new warnings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/kernel/lib/libcxx/include/__utility/declval.h b/kernel/lib/libcxx/include/__utility/declval.h
new file mode 100644
index 0000000..95e8159
--- /dev/null
+++ b/kernel/lib/libcxx/include/__utility/declval.h
@@ -0,0 +1,20 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the BastionOS freestanding C++ standard library.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBBASTION_UTILITY_DECLVAL_H
+#define _LIBBASTION_UTILITY_DECLVAL_H
+
+#include <__config>
+
+_LIBBASTION_BEGIN_NAMESPACE_STD
+
+template<class _Tp>
+__add_rvalue_reference(_Tp) declval() noexcept;
+
+_LIBBASTION_END_NAMESPACE_STD
+
+#endif // _LIBBASTION_UTILITY_DECLVAL_H