add PFFFT target, FFTS sources

FFTS/.gitignore

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+.DS_Store
+/.build
+/Packages
+xcuserdata/
+DerivedData/
+.swiftpm/configuration/registries.json
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+.netrc

FFTS/Package.swift

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+// swift-tools-version: 6.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "FFTS",
+    products: [
+        // Products define the executables and libraries a package produces, making them visible to other packages.
+        .library(
+            name: "FFTS",
+            targets: ["FFTS"]),
+    ],
+    targets: [
+      .plugin(name: "CMakePlugin", capability: .buildTool()),
+        .target(
+          name: "FFTS",
+        plugins: ["CMakePlugin"]),
+        .testTarget(
+            name: "FFTSTests",
+            dependencies: ["FFTS"]
+        ),
+    ]
+)

FFTS/Plugins/CMakePlugin/CMakePlugin.swift

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+import PackagePlugin
+
+@main

FFTS/Tests/FFTSTests/FFTSTests.swift

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+import Testing
+@testable import FFTS
+
+@Test func example() async throws {
+    // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+}