fix(libc): set EILSEQ in btowc for bytes that cannot start a character

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01SNqSvnFwRBz3D76n476yw6

core/libs/prx/libc/src/RuneLocale.cpp

@@ -1,4 +1,5 @@
 #include "prx/libc/include/general/VabiMacros.hpp"
+#include <cerrno>
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
@@ -148,7 +149,10 @@ std::uint32_t APS5_VABI towupper_nid_postfix(std::uint32_t c) {
 
 std::uint32_t APS5_VABI btowc_nid_postfix(int c) {
     const auto byte = static_cast<unsigned char>(c);
-    return c == -1 || byte >= 0x80 ? GuestRune::Weof : byte;
+    if (c == -1) return GuestRune::Weof;
+    if (byte < 0x80) return byte;
+    if (byte < 0xc0 || byte >= 0xf0) errno = 86;
+    return GuestRune::Weof;
 }
 
 int APS5_VABI wctob_nid_postfix(std::uint32_t c) {

core/libs/tests/GuestRuneLocale.cpp

@@ -149,5 +149,15 @@ int main() {
     }
     errno = 0;
     Require(towctrans_nid_postfix('A', 0) == 'A' && towctrans_nid_postfix('a', 3) == 'a' && errno == 0);
+    for (int c = -300; c < 0x1200; ++c) {
+        const auto byte = static_cast<unsigned char>(c);
+        errno = 0;
+        const std::uint32_t wide = btowc_nid_postfix(c);
+        const bool invalidLead = c != -1 && ((byte >= 0x80 && byte < 0xc0) || byte >= 0xf0);
+        Require(wide == (c != -1 && byte < 0x80 ? byte : 0xffffffffu) && errno == (invalidLead ? 86 : 0));
+    }
+    errno = 7;
+    Require(btowc_nid_postfix(0xc3) == 0xffffffff && btowc_nid_postfix(0xef) == 0xffffffff && btowc_nid_postfix(-1) == 0xffffffff && errno == 7);
+    Require(btowc_nid_postfix(0x41) == 'A' && errno == 7);
     return 0;
 }