Keep real staged compile errors navigable under warning floods

tests/build_diagnostics_tests.cpp

@@ -50,6 +50,25 @@ void contracts() {
         "compile", project);
     check(rows.size() == 2 && !rows[0].contains("file") && !rows[1].contains("file"),
           "External and traversing sources are never navigable");
+    const auto snapshot = fs::path("/cache/source-snapshots/verified/Scripts");
+    rows = editor::parse_build_diagnostics(
+        "/cache/source-snapshots/verified/Scripts/Nested/Game.cpp:21:3: error: broken\n"
+        "/cache/source-snapshots/verified/Scripts-other/Game.cpp:4:2: error: outside\n",
+        "compile", project, snapshot);
+    check(rows.size() == 2 && rows[0].at("file") == "Scripts/Nested/Game.cpp" &&
+              !rows[1].contains("file"),
+          "Only the verified staged Scripts subtree maps back to project source");
+    const auto nested_snapshot = project / ".faset/cache/source-snapshots/verified/Scripts";
+    rows = editor::parse_build_diagnostics(
+        "/project/.faset/cache/source-snapshots/verified/Scripts/Game.cpp:8:2: error: broken\n",
+        "compile", project, nested_snapshot);
+    check(rows.size() == 1 && rows[0].at("file") == "Scripts/Game.cpp",
+          "Project-contained build snapshot does not mask its verified Scripts mapping");
+    rows = editor::parse_build_diagnostics(
+        R"(C:\cache\snapshots\verified\Scripts\Game.cpp(17,4): error C2143: syntax error)" "\n",
+        "compile", windows, path_from_utf8(R"(C:\cache\snapshots\verified\Scripts)"));
+    check(rows.size() == 1 && rows[0].at("file") == "Scripts/Game.cpp",
+          "Windows staged source paths keep drive and Unicode navigation support");
     rows = editor::parse_build_diagnostics(
         "Scripts/../Scripts/Game.cpp:4:2: error: disguised traversal\n"
         "Scripts/Game.cpp:4:0: error: invalid column\n",

tests/build_schema_tests.cpp

@@ -232,6 +232,16 @@ int test_main(int argc, char** argv) {
                   compile_failed.log.find("fixture compile failure") != std::string::npos,
               "Failed compiler output retains raw log and structured source location");
         fs::remove(config.project_root / "emit-clang-error-and-fail-build");
+        atomic_write(config.project_root / "flood-warnings-and-fail-build", "fixture\n");
+        const auto flooded = builds.wait(builds.start_build());
+        bool retained_error = false;
+        for (const auto& diagnostic : flooded.diagnostics)
+            retained_error |= diagnostic.value("severity", "") == "error" &&
+                              diagnostic.value("file", "") == "Scripts/Gameplay.cpp";
+        check(flooded.state == "failed" && flooded.diagnostics.size() <= 200 &&
+                  retained_error && read_text(last_build) == previous_pointer,
+              "Compiler errors survive bounded third-party warning floods");
+        fs::remove(config.project_root / "flood-warnings-and-fail-build");
         atomic_write(config.project_root / "fail-unparseable-build", "fixture\n");
         const auto generic_failed = builds.wait(builds.start_build());
         check(generic_failed.state == "failed" &&

tests/build_schema_tool.cpp

@@ -40,6 +40,14 @@ int tool_main(int argc, char** argv) {
             return 0;
         }
         if (argc > 2 && std::string_view(argv[1]) == "--build") {
+            if (fs::exists("flood-warnings-and-fail-build")) {
+                for (int index = 0; index < 250; ++index)
+                    std::cerr << "/external/library.cpp:1:1: warning: dependency warning "
+                              << index << '\n';
+                std::cerr << path_to_utf8(fs::current_path() / "Scripts/Gameplay.cpp")
+                          << ":7:3: error: gameplay error after warnings\n";
+                return 1;
+            }
             if (fs::exists("emit-clang-error-and-fail-build")) {
                 std::cerr << path_to_utf8(fs::current_path() / "Scripts/Gameplay.cpp")
                           << ":7:3: error: fixture compile failure\n";

tests/build_service_tests.cpp

@@ -345,6 +345,13 @@ int integration(const fs::path& root) {
                  source + "\n#error intentional_build_failure\n");
     auto failed = service.wait(service.start_build());
     require(failed.state == "failed", "Invalid user C++ must fail build");
+    bool navigable_cpp_error = false;
+    for (const auto& diagnostic : failed.diagnostics)
+        navigable_cpp_error |= diagnostic.value("severity", "") == "error" &&
+                               diagnostic.value("file", "") == "Scripts/Gameplay.cpp" &&
+                               diagnostic.value("line", 0) > 0;
+    require(navigable_cpp_error,
+            "Real staged C++ compile failure maps to a navigable project source");
     require(read_text(config.cache_root / "last_build.json") == last,
             "Failed compile preserved last good build");
     atomic_write(config.project_root / "Scripts" / "Gameplay.cpp", source);