Add Brewfile and local formula for KDSingleApplication-qt6

This commit introduces a Brewfile for managing dependencies required by the Strawberry Music Player on macOS, including build tools and runtime dependencies. Additionally, a local Homebrew formula for KDSingleApplication-qt6 is added to facilitate its installation, as it is not consistently available in Homebrew core. A README is also included to guide users on installation and usage.

Formula/kdsingleapplication-qt6.rb

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+class KdsingleapplicationQt6 < Formula
+  desc "Helper class for single-instance Qt applications (Qt 6 build)"
+  homepage "https://github.com/KDAB/KDSingleApplication"
+  url "https://github.com/KDAB/KDSingleApplication/archive/refs/tags/v1.1.0.tar.gz"
+  sha256 "1f19124c0aa5c6fffee3da174f7d2e091fab6dca1e123da70bb0fe615bfbe3e8"
+  license "MIT"
+
+  depends_on "cmake" => :build
+  depends_on "ninja" => :build
+  depends_on "qt"
+
+  def install
+    args = std_cmake_args + %W[
+      -GNinja
+      -DKDSingleApplication_QT6=ON
+      -DKDSingleApplication_TESTS=OFF
+      -DKDSingleApplication_EXAMPLES=OFF
+      -DKDSingleApplication_DOCS=OFF
+      -DKDSingleApplication_DEVELOPER_MODE=OFF
+      -DKDSingleApplication_STATIC=OFF
+    ]
+
+    system "cmake", "-S", ".", "-B", "build", *args
+    system "cmake", "--build", "build"
+    system "cmake", "--install", "build"
+  end
+
+  test do
+    # Verify CMake package is usable via find_package(KDSingleApplication-qt6 CONFIG REQUIRED)
+    (testpath/"CMakeLists.txt").write <<~CMAKE
+      cmake_minimum_required(VERSION 3.16)
+      project(kdsa_test LANGUAGES CXX)
+      find_package(KDSingleApplication-qt6 CONFIG REQUIRED)
+      add_executable(test_kdsa main.cpp)
+      target_link_libraries(test_kdsa PRIVATE KDAB::kdsingleapplication)
+    CMAKE
+
+    (testpath/"main.cpp").write <<~CPP
+      #include <QCoreApplication>
+      int main(int argc, char** argv) {
+        QCoreApplication app(argc, argv);
+        return 0;
+      }
+    CPP
+
+    system "cmake", "-S", ".", "-B", "build",
+           "-DCMAKE_PREFIX_PATH=#{opt_prefix}"
+    system "cmake", "--build", "build"
+  end
+end
+

Formula/kdsingleapplication-qt6/README.md

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+# kdsingleapplication-qt6 (local Homebrew formula)
+
+This directory exists to keep any supporting files for the local Homebrew formula
+next to it (e.g. patches or notes).
+
+## Install (from this Strawberry repo)
+
+From the repo root:
+
+```bash
+brew tap strawberry/local "file://$PWD"
+brew install strawberry/local/kdsingleapplication-qt6
+```
+
+## Why it exists
+
+Strawberry’s build requires the CMake package `KDSingleApplication-qt6`, but it is
+not consistently available via Homebrew core. Shipping a local formula makes the
+dependency easy to install for anyone building this repo on macOS.
+
+## Note for local development
+
+Homebrew taps are Git clones, so the formula must be committed (or pushed to a remote)
+to be visible to `brew tap`.
+