fix(toolchain): msvc@system is located for a build program, never installed

On a Windows machine whose default is msvc@system, a build program of a
bare cross target resolved its host toolchain through the package index
and looked for an msvc@system package. The spec now resolves the way a
native build resolves it: the Visual Studio installation is located, and
its absence is reported with the installation guidance. E2E 1017 builds
openkal's 32-bit layout probe shape on Windows with msvc@system.

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>

Author: Sunrisepeak

src/build/prepare/xlings.cpp

@@ -18,6 +18,8 @@ import mcpp.manifest;
 import mcpp.source_kind;
 import mcpp.toolchain.hostflags;   // the compile-token producer the package std module reuses
 import mcpp.toolchain.detect;
+import mcpplibs.os.platform;
+import mcpp.toolchain.msvc;
 import mcpp.toolchain.dialect;
 import mcpp.toolchain.fingerprint;
 import mcpp.toolchain.registry;
@@ -235,6 +237,27 @@ step3_define_host_tc_closures_and_refresh_index(PrepareState& state) {
             return std::unexpected(std::format(
                 "toolchain spec '{}' is invalid for the build.mcpp host resolve", hostSpecText));
         }
+        // `<family>@system` NAMES THIS MACHINE'S INSTALLATION, AND IS LOCATED,
+        // NEVER INSTALLED (2026.11.7.1). A Windows machine whose default is
+        // `msvc@system`, building for a bare cross target, handed that spec to
+        // the package resolver below, which looked for `msvc@system` in the
+        // index (design 2026-11-07 §4.1.3). The build program is located the
+        // way a native build locates the same spec.
+        if (mcpp::toolchain::origin_of(*spec) == mcpp::toolchain::Origin::SystemMsvc) {
+            if constexpr (!mcpplibs::os::is_windows) {
+                return std::unexpected(std::format(
+                    "host toolchain for build.mcpp ('{}') is only available on Windows hosts", hostSpecText));
+            }
+            auto inst = mcpp::toolchain::msvc::detect_installation();
+            if (!inst) return std::unexpected(mcpp::toolchain::msvc::install_guidance());
+            auto htc = mcpp::toolchain::detect(
+                inst->clPath, state.runtimePayload, state.runtimeBindingSnapshot.contractHash);
+            if (!htc) return std::unexpected(htc.error().message);
+            mcpp::ui::info("Resolved", std::format(
+                "host toolchain for build.mcpp: {} → {}", hostSpecText, htc->label()));
+            state.hostTcCache = std::pair{inst->clPath, *htc};
+            return std::pair{state.hostTcCache->first, as_host(state.hostTcCache->second)};
+        }
         // Deliberately NO target injection: the spec resolves for the host.
         auto pkg = mcpp::toolchain::to_xim_package(*spec);
         auto cfgH = state.get_cfg(true);

tests/e2e/1017_a_system_msvc_builds_the_build_program_of_a_cross_target.sh

@@ -0,0 +1,61 @@
+#!/usr/bin/env bash
+# requires: windows msvc llvm
+# `msvc@system` IS LOCATED FOR A BUILD PROGRAM, NEVER INSTALLED (2026.11.7.1).
+# On a Windows machine whose default toolchain is `msvc@system`, a project with
+# a build program built for a bare cross target resolved the build program's
+# host toolchain through the package index and looked for an `msvc@system`
+# package there (openkal's 32-bit layout probe, design 2026-11-07 §4.1.3). The
+# build program is now compiled by the located Visual Studio, as a native build
+# of the same machine is, and its directive reaches the cross build.
+set -e
+
+CONF="${MCPP_HOME:-$HOME/.mcpp}/config.toml"
+ORIG_DEFAULT=""
+if [[ -f "$CONF" ]]; then
+    ORIG_DEFAULT=$(sed -n '/^\[toolchain\]/,/^\[/p' "$CONF" \
+        | grep -E '^default[[:space:]]*=' | head -1 | cut -d'"' -f2 || true)
+fi
+TMP=$(mktemp -d)
+restore() {
+    if [[ -n "$ORIG_DEFAULT" ]]; then
+        "$MCPP" toolchain default "$ORIG_DEFAULT" >/dev/null 2>&1 || true
+    fi
+    rm -rf "$TMP"
+}
+trap restore EXIT
+
+"$MCPP" toolchain default msvc >/dev/null 2>&1 \
+    || { echo "FAIL: msvc@system not selectable"; exit 1; }
+
+mkdir -p "$TMP/probe/src" "$TMP/probe/inc"
+cd "$TMP/probe"
+cat > mcpp.toml <<'EOF2'
+[package]
+name    = "layout"
+version = "0.1.0"
+EOF2
+cat > build.mcpp <<'EOF2'
+#include <cstdio>
+int main() {
+    std::puts("mcpp:include-dir=inc");
+    std::puts("mcpp:rerun-if-changed=build.mcpp");
+    return 0;
+}
+EOF2
+cat > inc/layout.h <<'EOF2'
+struct pair_layout { unsigned size; unsigned long long stamp; };
+EOF2
+cat > src/main.cpp <<'EOF2'
+#include "layout.h"
+static_assert(__builtin_offsetof(pair_layout, stamp) == 8);
+extern "C" void _start() { for (;;) {} }
+EOF2
+
+out=$("$MCPP" build --target riscv64-none-elf 2>&1) \
+    || { echo "FAIL: the cross build with a build program failed:"; echo "$out"; exit 1; }
+grep -q "host toolchain for build.mcpp: msvc@system" <<< "$out" \
+    || { echo "FAIL: the build program was not compiled by the located msvc@system:"; echo "$out"; exit 1; }
+if grep -qi "xim:msvc@system\|msvc@system.*not found in the" <<< "$out"; then
+    echo "FAIL: msvc@system was looked up as a package:"; echo "$out"; exit 1
+fi
+echo "OK: msvc@system compiled the build program of a riscv64-none-elf build"