feat(syscall): add x86_64 raw syscall layer, errno, internal ABI headers

src/internal/libc.h

@@ -0,0 +1,15 @@
+#ifndef VLIBC_INTERNAL_LIBC_H
+#define VLIBC_INTERNAL_LIBC_H
+
+/*
+ * vlibc — internal compiler-attribute shorthand.
+ *
+ * Everything internal to the library is declared with hidden visibility so
+ * that neither the static archive nor the shared library exports symbols
+ * outside the public API surface. The shared-library case matters most:
+ * without `hidden`, every internal helper would become a dynamic symbol that
+ * could be interposed or collide with a consumer's own symbols.
+ */
+#define hidden __attribute__((__visibility__("hidden")))
+
+#endif /* VLIBC_INTERNAL_LIBC_H */