feat: make kernel architecture-agnostic; primitive work on arm port

kernel/src/Arch/Arch.hpp

@@ -0,0 +1,25 @@
+/*
+    * Arch.hpp
+    * Target architecture selection
+    * Copyright (c) 2026 Daniel Hammer
+*/
+
+#pragma once
+
+// Everything outside src/Arch/ is architecture-neutral and talks to the CPU
+// only through the interfaces in src/Arch/*.hpp. Each supported architecture
+// implements them under src/Arch/<arch>/; the build compiles only the
+// directory matching ARCH. See docs/porting.md.
+//
+// Interface headers that need inline (per-arch) definitions include the
+// matching implementation header at their end, selected below.
+
+#if defined(__x86_64__)
+#define MONTAUK_ARCH_X86_64 1
+#define MONTAUK_ARCH_NAME "x86_64"
+#elif defined(__aarch64__)
+#define MONTAUK_ARCH_AARCH64 1
+#define MONTAUK_ARCH_NAME "aarch64"
+#else
+#error "MontaukOS: unsupported target architecture (add one under src/Arch/)"
+#endif