fix(toolchain): 'default msvc' clears the target axis; e2e 99 asserts the resolved family

A stale [toolchain].default_target (left by a previous mingw/musl default)
survived 'toolchain default msvc' — the msvc branch persisted only the
toolchain axis. The leftover x86_64-windows-gnu then hijacked the next
build: the target's convention pin overrode msvc@system and cl.exe builds
silently ran gcc instead (caught by ci-windows: e2e 99 found .o instead of
.obj after a 9s 'msvc' build). msvc default now writes default_target=''
like every other default; e2e 99 additionally asserts the resolved family
and uses the boundary-safe default-key grep (as 95/97 already do).

Author: Sunrisepeak

src/toolchain/lifecycle.cppm

@@ -647,6 +647,15 @@ export int toolchain_set_default(const mcpp::config::GlobalConfig& cfg,
                 mcpp::ui::error(wr.error().message);
                 return 1;
             }
+            // Clear the target axis too: a stale default_target (e.g.
+            // x86_64-windows-gnu left by a previous mingw default) would
+            // otherwise hijack the next build — the target's convention pin
+            // overrides the toolchain, silently building with gcc instead
+            // of the just-selected cl.exe. Caught by ci-windows e2e 99.
+            if (auto wt = mcpp::config::write_default_target(cfg, ""); !wt) {
+                mcpp::ui::error(wt.error().message);
+                return 1;
+            }
             mcpp::ui::status("Default", std::format(
                 "set to msvc@system (was: {})",
                 cfg.defaultToolchain.empty() ? "<none>" : cfg.defaultToolchain));

tests/e2e/99_msvc_native_build.sh

@@ -9,8 +9,10 @@ set -e
 CONF="${MCPP_HOME:-$HOME/.mcpp}/config.toml"
 ORIG_DEFAULT=""
 if [[ -f "$CONF" ]]; then
+    # NB: match `default =` exactly — `default_target =` also starts with
+    # "default" (the persisted pair since the naming unification).
     ORIG_DEFAULT=$(sed -n '/^\[toolchain\]/,/^\[/p' "$CONF" \
-        | grep '^default' | head -1 | cut -d'"' -f2 || true)
+        | grep -E '^default[[:space:]]*=' | head -1 | cut -d'"' -f2 || true)
 fi
 TMP=$(mktemp -d)
 restore() {
@@ -42,6 +44,10 @@ int main() { std::println("{}", hello::greet()); return 0; }
 EOF
 
 out=$("$MCPP" build 2>&1) || { echo "FAIL: msvc build: $out"; exit 1; }
+# the build must actually resolve msvc — a stale default_target could
+# otherwise silently reroute to another toolchain (that exact bug shipped
+# once: mingw's leftover default_target hijacked `toolchain default msvc`)
+grep -q "msvc" <<<"$out" || { echo "FAIL: build did not resolve msvc: $out"; exit 1; }
 run_out=$("$MCPP" run 2>&1) || { echo "FAIL: msvc run: $run_out"; exit 1; }
 [[ "$run_out" == *"cl-ok"* ]] || { echo "FAIL: run output: $run_out"; exit 1; }