test(core): Swift oracle — the filius-ipad core built on Linux as the reference

tools/swift-oracle builds the unchanged Swift runtime (pinned 3b5ded0)
in Docker and replays JSON scenarios; the TypeScript engine must produce
the same trace events, results and end state. Five scenarios, 884
events, identical.

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

tools/swift-oracle/Package.swift

@@ -0,0 +1,23 @@
+// swift-tools-version:5.9
+// The filius-ipad core (runtime, model, persistence, reports) built on Linux as
+// a reference for the TypeScript port. Sources/FiliusCore is filled by
+// fetch-upstream.sh from a pinned commit; nothing of it is checked in here.
+import PackageDescription
+
+let package = Package(
+    name: "FiliusOracle",
+    dependencies: [.package(url: "https://github.com/apple/swift-crypto.git", from: "3.0.0")],
+    targets: [
+        // Apple-only modules the core imports, reduced to what it uses.
+        .target(name: "CoreGraphics"),
+        .target(name: "CryptoKit", dependencies: [.product(name: "Crypto", package: "swift-crypto")]),
+        .systemLibrary(name: "CZlib"),
+        .target(name: "Compression", dependencies: ["CZlib"]),
+        .target(
+            name: "FiliusCore",
+            dependencies: ["CoreGraphics", "CryptoKit", "Compression"],
+            swiftSettings: [.unsafeFlags(["-enable-testing"])]
+        ),
+        .executableTarget(name: "Oracle", dependencies: ["FiliusCore"]),
+    ]
+)