test(toolchain): make musl frontend assertions host-aware

B1 changed the musl frontend candidate list on Windows hosts (.exe first), and
these two assertions pinned the bare spelling as front(). They pass on Linux
either way, which is exactly why the break only showed up in Windows CI — the
same blind spot that let B1 itself survive.

Adds expected_musl_frontend() next to the existing expected_musl_xim() helper,
mirroring how the mingw test already handles the host split. The helper states
why .exe comes first rather than leaving a bare constant.

Author: Sunrisepeak

tests/unit/test_toolchain_registry.cpp

@@ -18,6 +18,15 @@ static std::string expected_musl_xim() {
     else                                    return host_musl() + "-gcc";
 }
 
+// Frontend candidates are host-aware for the same reason the mingw ones are:
+// they are resolved with filesystem::exists, and on a Windows host the file on
+// disk is `<triple>-g++.exe`. The .exe spelling comes first so it wins on a
+// case-insensitive filesystem where both would match.
+static std::string expected_musl_frontend(const std::string& triple) {
+    if constexpr (mcpp::platform::is_windows) return triple + "-g++.exe";
+    else                                      return triple + "-g++";
+}
+
 // ── canonical two-axis identity ──────────────────────────────────────────────
 
 TEST(ToolchainRegistry, MapsGccSpecToGccPackage) {
@@ -57,7 +66,7 @@ TEST(ToolchainRegistry, LegacyMuslSuffixNormalizesToMuslTarget) {
     EXPECT_EQ(pkg.ximName, expected_musl_xim());
     EXPECT_EQ(pkg.ximVersion, "15.1.0");
     ASSERT_FALSE(pkg.frontendCandidates.empty());
-    EXPECT_EQ(pkg.frontendCandidates.front(), host_musl() + "-g++");
+    EXPECT_EQ(pkg.frontendCandidates.front(), expected_musl_frontend(host_musl()));
     EXPECT_FALSE(pkg.needsGccPostInstallFixup);
 }
 
@@ -73,7 +82,7 @@ TEST(ToolchainRegistry, CrossArchMuslTargetPicksTripleNamedPackage) {
     auto pkg = to_xim_package(spec);
     EXPECT_EQ(pkg.ximName, spec.target.str() + "-gcc");
     ASSERT_FALSE(pkg.frontendCandidates.empty());
-    EXPECT_EQ(pkg.frontendCandidates.front(), spec.target.str() + "-g++");
+    EXPECT_EQ(pkg.frontendCandidates.front(), expected_musl_frontend(spec.target.str()));
     EXPECT_FALSE(pkg.needsGccPostInstallFixup);
 }