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, 42 insertions, 0 deletions

diff --git a/kernel/lib/libcxx/include/__tuple/tuple_element.h b/kernel/lib/libcxx/include/__tuple/tuple_element.h
new file mode 100644
index 0000000..97b347f
--- /dev/null
+++ b/kernel/lib/libcxx/include/__tuple/tuple_element.h
@@ -0,0 +1,42 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the BastionOS freestanding C++ standard library.
+//
+// tuple_size and tuple_element — protocol types for structured bindings.
+// Primary templates here; specializations in pair.h, array, tuple.h, etc.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBBASTION_TUPLE_TUPLE_ELEMENT_H
+#define _LIBBASTION_TUPLE_TUPLE_ELEMENT_H
+
+#include <__config>
+#include <cstddef>
+
+_LIBBASTION_BEGIN_NAMESPACE_STD
+
+// Primary templates (specialized by pair, array, tuple, variant).
+template<class _Tp>
+struct tuple_size; // : integral_constant<size_t, N>
+
+template<class _Tp>
+struct tuple_size<const _Tp> : tuple_size<_Tp> {};
+
+template<class _Tp>
+inline constexpr size_t tuple_size_v = tuple_size<_Tp>::value;
+
+template<size_t _Ip, class _Tp>
+struct tuple_element; // { using type = ...; }
+
+template<size_t _Ip, class _Tp>
+struct tuple_element<_Ip, const _Tp> {
+    using type = const typename tuple_element<_Ip, _Tp>::type;
+};
+
+template<size_t _Ip, class _Tp>
+using tuple_element_t = typename tuple_element<_Ip, _Tp>::type;
+
+_LIBBASTION_END_NAMESPACE_STD
+
+#endif // _LIBBASTION_TUPLE_TUPLE_ELEMENT_H