fix(doctor): plan to describe, not to build

doctor's prepare is offline, and now plan_only with a scratch work_dir, as
`mcpp emit build-database` plans: the std module is described rather than
compiled, generated files are compared rather than written, and the lock
goes to the scratch directory. e2e 22 relied on doctor precompiling std as a
side effect; it now asserts the cache is still empty after doctor and makes
the std entry it lists with a build.

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

Author: Sunrisepeak

src/doctor.cppm

@@ -206,7 +206,11 @@ export int doctor_report() {
     // diagnoses, and reports something else, is two defects.
     //
     // Offline for the call: nothing is downloaded, installed or replaced, so
-    // a build that would install first is reported as exactly that.
+    // a build that would install first is reported as exactly that. Planned
+    // to describe, not to build (`plan_only`, as `mcpp emit build-database`
+    // plans): the std module is described rather than compiled, generated
+    // files are compared rather than written, and the lock this planning
+    // produces goes to a scratch directory, not the project.
     const bool inPackage = mcpp::project::find_manifest_root(std::filesystem::current_path()).has_value();
     bool prepareWouldInstall = false;
     auto pctx = [&]() -> decltype(mcpp::build::prepare_build(false)) {
@@ -215,7 +219,13 @@ export int doctor_report() {
         const bool wasQuiet = mcpp::ui::is_quiet();
         mcpp::ui::set_quiet(true);
         (void)mcpp::build::refusal::take();
-        auto r = mcpp::build::prepare_build(/*print_fingerprint=*/false);
+        mcpp::build::BuildOverrides ov;
+        ov.plan_only = true;
+        std::error_code tec;
+        ov.work_dir = std::filesystem::temp_directory_path(tec)
+            / std::format("mcpp-doctor-{}", std::chrono::steady_clock::now().time_since_epoch().count());
+        auto r = mcpp::build::prepare_build(/*print_fingerprint=*/false, /*includeDevDeps=*/false, {}, ov);
+        std::filesystem::remove_all(ov.work_dir, tec);
         prepareWouldInstall = !r && mcpp::build::refusal::take()
             == mcpp::build::refusal::Code::OfflineDownloadRequired;
         mcpp::ui::set_quiet(wasQuiet);

tests/e2e/22_doctor_cache_publish.sh

@@ -12,10 +12,6 @@ trap "rm -rf $TMP" EXIT
 export MCPP_HOME="$TMP/mcpp-home"
 
 # 0) cache list on a genuinely empty cache: friendly message.
-#    This has to come BEFORE doctor — doctor resolves a build plan, which
-#    precompiles the std module, and `cache list` now reports std entries too.
-#    Hiding them is how 16 GB of duplicated std BMIs went unnoticed, so listing
-#    them is the fix, not a regression.
 out=$("$MCPP" cache list 2>&1)
 [[ "$out" == *"empty"* ]] || { echo "cache list on empty cache: '$out'"; exit 1; }
 
@@ -29,8 +25,17 @@ grep -q 'Checking registry'       doctor.log || { cat doctor.log; echo "no regis
 grep -q 'Checking cache health'   doctor.log || { cat doctor.log; echo "no cache check"; exit 1; }
 grep -q 'Doctor result'             doctor.log || { cat doctor.log; echo "no result line"; exit 1; }
 
-# 2) after doctor, the std module it precompiled must be VISIBLE. std entries
+# 1b) doctor changes nothing (2026.10.17.1): its plan is prepared offline and
+#     builds nothing, so the cache is still empty after it.
+out=$("$MCPP" cache list 2>&1)
+[[ "$out" == *"empty"* ]] || { echo "doctor wrote to the build cache: '$out'"; exit 1; }
+
+# 2) after a build, the std module it precompiled must be VISIBLE. std entries
 #    dominate the cache's size, and `cache list` used to walk only dep entries.
+#    Hiding them is how 16 GB of duplicated std BMIs went unnoticed, so listing
+#    them is the fix, not a regression.
+( cd "$TMP" && "$MCPP" new stdprobe >/dev/null && cd stdprobe && "$MCPP" build > "$TMP/stdprobe.log" 2>&1 ) \
+    || { cat "$TMP/stdprobe.log"; echo "the build that precompiles std failed"; exit 1; }
 out=$("$MCPP" cache list 2>&1)
 [[ "$out" == *"std"* ]] || { echo "cache list omits the std entry: '$out'"; exit 1; }
 # The age column must be plausible: file_time_type's epoch is not the Unix