test(avlivewire): end-to-end chunked loopback

Feeds 20 framed skeleton payloads through StreamDemuxer in
7-byte chunks (worst-case TCP fragmentation). Fixed a split
range operator from the plan that did not parse.

shared/AVLiveWire/Tests/AVLiveWireTests/LoopbackTests.swift

@@ -0,0 +1,37 @@
+import XCTest
+@testable import AVLiveWire
+
+/// Feeds an encoded frame stream through the demuxer in 7-byte
+/// chunks — the worst-case fragmentation a TCP tunnel can produce.
+final class LoopbackTests: XCTestCase {
+    func testManyFramesChunked() {
+        var skel = SkeletonPayload()
+        skel.valid[0] = true
+        skel.joints[0] = SIMD3(1, 1, 1)
+
+        var stream = Data()
+        for i in 0..<20 {
+            let payload = skel.encoded()
+            let h = FrameHeader(tag: .skeleton, pid: Int16(i),
+                timestamp: Double(i),
+                length: UInt32(payload.count))
+            stream += h.encoded() + payload
+        }
+
+        var demux = StreamDemuxer()
+        var frames: [StreamDemuxer.Frame] = []
+        var offset = 0
+        while offset < stream.count {
+            let end = min(offset + 7, stream.count)
+            let lo = stream.index(stream.startIndex, offsetBy: offset)
+            let hi = stream.index(stream.startIndex, offsetBy: end)
+            frames += demux.feed(Data(stream[lo..<hi]))
+            offset = end
+        }
+
+        XCTAssertEqual(frames.count, 20)
+        XCTAssertEqual(frames[5].header.pid, 5)
+        XCTAssertEqual(SkeletonPayload(decoding: frames[5].payload),
+                       skel)
+    }
+}