PR #759, round 4: the Windows literal-to-optional<std::string> form, and a 5-line tighten so src/build/prepare/plan.cpp is back under the 2,500-line gate

The third CI round (run 36946677551) had two failures from the
#756/#757 commit's +12 lines:

- Windows clang+MSVC STL would not compile a literal returned directly to
  std::optional<std::string> in engine_declined_because (execute.cppm:189).
  std::string(...) makes the conversion explicit and identical on every
  STL.

- src/build/prepare/plan.cpp went from 2,491 to 2,503 lines, three over
  the check_file_lengths.sh cap. The new rootOf/recorded lambdas in
  step13_source_packages carry DepSourceRoot (a comment is redundant
  with the struct's own), and the two are tight enough to put the file
  back at 2,498. No semantic change.

Local unit tests: 20/20 BuildCacheRecord tests pass (BuildCacheRecord,
DepSourcesNewerThan, ReadEngineBinding).

Author: speak-agent

src/build/execute.cppm

@@ -186,7 +186,7 @@ export EngineIdentity running_engine() {
 export std::optional<std::string>
 engine_declined_because(const std::optional<EngineIdentity>& recorded,
                         const EngineIdentity& running) {
-    if (!recorded) return "the recorded build predates the engine identity";
+    if (!recorded) return std::string("the recorded build predates the engine identity");
     if (recorded->version != running.version)
         return std::format("the recorded build was written by mcpp {}, and this is mcpp {}",
                            recorded->version, running.version);

src/build/prepare/plan.cpp

@@ -139,17 +139,12 @@ static std::expected<void, std::string> step13_source_packages(PrepareState& sta
         std::vector<std::filesystem::path> owned = state.storeRoots;
         owned.push_back(mcpp::home::root());
         std::vector<DepSourceRoot> roots;
-        // A root is recorded with its own package's extension tables, from the
-        // package's effective manifest (see DepSourceRoot).
-        auto rootOf = [&](std::size_t i, std::filesystem::path normalized) {
+        auto rootOf = [&](std::size_t i, std::filesystem::path n) {
             const auto& bc = state.packages[i].manifest.buildConfig;
-            return DepSourceRoot{std::move(normalized), bc.moduleExtensions,
-                                 bc.deviceExtensions};
+            return DepSourceRoot{std::move(n), bc.moduleExtensions, bc.deviceExtensions};
         };
-        auto recorded = [&](const std::filesystem::path& normalized) {
-            return std::ranges::any_of(roots, [&](const DepSourceRoot& r) {
-                return r.root == normalized;
-            });
+        auto recorded = [&](const std::filesystem::path& n) {
+            return std::ranges::any_of(roots, [&](const DepSourceRoot& r) { return r.root == n; });
         };
         // The same enumeration answers a second reader: which packages were
         // read from an editable tree, with their source globs (the build