feat: add Fn hotkey monitor and probe CLI

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

Sources/SstCore/FnKeyTracker.swift

@@ -0,0 +1,12 @@
+public struct FnKeyTracker {
+    private var isDown = false
+
+    public init() {}
+
+    /// Feed the current Fn flag state; returns an event only on a transition.
+    public mutating func process(fnDown: Bool) -> HotkeyEvent? {
+        guard fnDown != isDown else { return nil }
+        isDown = fnDown
+        return fnDown ? .pressed : .released
+    }
+}