Package Effector for Arch and give it a desktop entry

packaging/arch/PKGBUILD builds the tagged release from this repository
and installs it with pacman; the version test keeps its pkgver in step
with CMakeLists.txt. A desktop entry puts the interface in the app menu.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core_tests.cpp

@@ -1725,6 +1725,13 @@ int main() {
         check(newest != std::string_view::npos &&
                   log.substr(newest + 5).starts_with(std::string(kVersion) + "]"),
               "the newest changelog entry is this version");
+        std::ifstream pkgbuild(std::string(EFFECTOR_SOURCE_DIR) + "/packaging/arch/PKGBUILD");
+        std::string line;
+        std::string pkgver;
+        while (std::getline(pkgbuild, line)) {
+            if (line.starts_with("pkgver=")) pkgver = line.substr(7);
+        }
+        check(pkgver == kVersion, "the Arch PKGBUILD packages this version");
         const auto options = parsedOptions({"--version"});
         check(options && options->showVersion, "--version is understood");
         const auto shortOptions = parsedOptions({"-V"});