fix(prepare): a version conflict on a package with no C++ module says so

Two versions of one package are reconciled by mangling the secondary
copy's module names. The two branches that refuse name both versions and
who asked for them; the third reported only that the package declares no
named C++ module to rewrite, which is a true statement about a package
the reader never asked to be staged and says nothing about the conflict
that brought it there. A C package reaches it: compat.vulkan-runtime
compiles one C file, so a manifest that pins one version of it while a
dependency asks for another got a message about modules.

It now names both versions, both requesters, and what to do.

examples/10 moves its pin to compat.vulkan-runtime 2026.09.06, the
version compat.vulkan asks for since mcpp-index#350. The example is the
consumer that found this.

Author: speak-agent

.agents/docs/2026-09-05-heterogeneous-verify.sh

@@ -31,9 +31,11 @@ got=$("$STORE" --version 2>&1 | head -1)
 # value was written, so both tools are asked what they hold afterwards.
 "$STORE" self config --mirror "${MCPP_VERIFY_MIRROR:-CN}" >/dev/null 2>&1 || true
 "$XL" config --mirror "${MCPP_VERIFY_MIRROR:-CN}" >/dev/null 2>&1 || true
-# Both tools print their configuration banner on stderr, so a `2>/dev/null`
-# here reads an empty string and reports a mirror that is in fact set.
-xm=$("$XL" config 2>&1 | sed 's/\x1b\[[0-9;]*m//g' | grep -i 'mirror' | head -1 | awk '{print $NF}')
+# THE SETTING, NOT THE TOOL'S OUTPUT. `xlings config` renders a banner through
+# its ui layer, which prints nothing when its stdout is a pipe -- so a command
+# substitution reads an empty string and reports a mirror that is in fact set.
+# The file the tool writes answers the same question and cannot be suppressed.
+xm=$(python3 -c "import json,os;print(json.load(open(os.path.expanduser('~/.xlings/.xlings.json'))).get('mirror',''))" 2>/dev/null)
 [ "$xm" = "${MCPP_VERIFY_MIRROR:-CN}" ] && ok "xlings mirror is $xm" || fail "xlings mirror is '$xm', not ${MCPP_VERIFY_MIRROR:-CN}"
 mm=$("$STORE" self config 2>&1 | sed 's/\x1b\[[0-9;]*m//g' | grep -i 'mirror' | head -1 | awk '{print $NF}')
 [ -n "$mm" ] && ok "mcpp mirror is $mm" || printf 'note: mcpp self config does not print its mirror (%s)\n' "$("$STORE" self config 2>&1 | head -1)"
@@ -142,6 +144,13 @@ if [ -n "$SRC" ] && [ -d "$SRC/examples/10-vulkan-compute/app" ]; then
         || fail "example 10 --no-accel: $(printf '%s' "$out" | tail -3 | tr '\n' ' ')"
     hs=$(ls "$HOME"/.mcpp/registry/data/xpkgs/compat-x-vulkan-runtime/*/mcpp_generated/vulkan_runtime/HOST-SURFACE.txt \
             "$ex10"/.mcpp/.xlings/data/xpkgs/compat-x-vulkan-runtime/*/mcpp_generated/vulkan_runtime/HOST-SURFACE.txt 2>/dev/null | head -1)
+    # WHICH ADAPTER WROTE IT. The substitution criteria below are properties of
+    # 2026.09.06, the version that declares its payload set; an older report
+    # carries neither the declarations nor the class they produce, so both
+    # would pass on an empty finding. The version is a path segment.
+    adapter=$(printf '%s' "$hs" | sed -n 's#.*/compat-x-vulkan-runtime/\([^/]*\)/.*#\1#p')
+    [ "$adapter" = "${MCPP_VERIFY_ADAPTER:-2026.09.06}" ] && ok "the farm was written by compat.vulkan-runtime $adapter" \
+        || fail "the farm was written by compat.vulkan-runtime '$adapter', not ${MCPP_VERIFY_ADAPTER:-2026.09.06}"
     if [ -n "$hs" ]; then
         printf -- '--- %s\n' "$hs"; sed -n '/^## farmed/,$p' "$hs" | head -60
         # WHAT A SANDBOX CAN AND CANNOT ASSERT. A subos shares the host's

examples/10-vulkan-compute/app/mcpp.toml

@@ -22,7 +22,7 @@ plugins = { version = "0.1.0", features = ["rules-spirv"], host-module = true }
 # below is a payload and the hardware ones are the host's.
 [dependencies.compat]
 vulkan         = "1.4.357.0"
-vulkan-runtime = "2026.09.05"
+vulkan-runtime = "2026.09.06"
 
 # The two payloads this build names rather than discovers: the shader compiler,
 # and a Vulkan driver that is always present because it is the CPU. With

src/build/prepare.cppm

@@ -5554,7 +5554,27 @@ prepare_build(bool print_fingerprint,
                     // same map in both the secondary and its consumer.
                     auto moduleNames = declared_modules_for(
                         secondaryRoot, secondaryManifest);
-                    if (!moduleNames) return std::unexpected(moduleNames.error());
+                    // The two branches above name both versions and who asked
+                    // for them; this one used to report only that the package
+                    // declares no named C++ module, which is a true statement
+                    // about a package the reader never asked to be staged. A
+                    // C package -- compat.vulkan-runtime is one -- reaches
+                    // here whenever a manifest pins one version of it and
+                    // another dependency asks for a second, and the message
+                    // has to say that before it says anything about modules.
+                    if (!moduleNames) return std::unexpected(std::format(
+                        "dependency '{}{}{}' has irreconcilable versions:\n"
+                        "  '{}' requested by '{}'\n"
+                        "  '{}' requested by '{}'\n"
+                        "Multi-version mangling cannot separate them: {}.\n"
+                        "A package with no named C++ module has nothing to "
+                        "rewrite, so the two requests must agree. Align the "
+                        "pin in your mcpp.toml with the version the other "
+                        "dependency asks for.",
+                        key.ns, key.ns.empty() ? "" : ".", key.shortName,
+                        it->second.version, it->second.requestedBy,
+                        spec.version, item.requestedBy,
+                        moduleNames.error()));
                     std::map<std::string, std::string> rename;
                     for (auto const& module : *moduleNames) {
                         rename.emplace(module,