test(wasm): the cases the review found

WasmBuildRefusals checks that -flto in compileFlags and in linkFlags,
-fno-exceptions and a shared library are each refused with their
cause, and that none of them leaves bin/ or build/ behind. With the
check disabled, each of them failed on a missing std.cppm instead,
after the build directories had been made.

WasiSysrootWithoutExceptions also takes its sysroot through missing
and without the target's directory, each with a message of its own,
checks that a refused build leaves nothing behind, and that a static
library passes without libunwind.

WasmExceptions catches an exception thrown inside libc++.a (stoi).
vector::at throws from a header compiled into the fixture, so the
fixture also passed against the libc++ built without exceptions. Its C
source uses setjmp, which the old C flags did not compile. An
exception that escapes main has to end the run as wasmtime's uncaught
exception, where any nonzero exit used to pass, a trap or a timeout
included.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/WasmExceptions/fixture/jump.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+#include <setjmp.h>
+#include <stdint.h>
+
+static jmp_buf target;
+
+static void JumpBack(int32_t value) {
+    longjmp(target, value);
+}
+
+// setjmp's value may only be compared or switched on, never stored.
+int32_t JumpRoundTrip(void) {
+    switch (setjmp(target)) {
+    case 0:
+        JumpBack(7);
+        return 0;
+    case 7:
+        return 7;
+    default:
+        return -1;
+    }
+}

tests/WasmExceptions/fixture/main.cpp

@@ -3,6 +3,9 @@
 
 import std;
 
+// jump.c, compiled as C.
+extern "C" std::int32_t JumpRoundTrip();
+
 namespace {
     std::int32_t unwound = 0;
 
@@ -31,5 +34,13 @@ int main(int argc, char** argv) {
     } catch (const std::out_of_range&) {
         std::println("caught: out_of_range from libc++");
     }
+    // vector::at throws from a header, compiled into this unit; stoi throws
+    // inside libc++.a, which therefore has to be the exception-enabled one.
+    try {
+        std::println("unreachable: {}", std::stoi("not a number"));
+    } catch (const std::invalid_argument&) {
+        std::println("caught: invalid_argument from inside libc++");
+    }
+    std::println("longjmp came back with {}", JumpRoundTrip());
     return 0;
 }