feat(avlivewire): skeleton and video codecs

Add SkeletonPayload (91 ARKit joints + per-joint validity) and
VideoPayload (one HEVC access unit + keyframe flag) with
big-endian encode/decode. Reuses Task 2 Data/UInt32 helpers.

shared/AVLiveWire/Sources/AVLiveWire/WirePayloads.swift

@@ -0,0 +1,65 @@
+import Foundation
+import simd
+
+/// 91 ARKit joints in world space + a per-joint validity flag.
+public struct SkeletonPayload: Equatable {
+    public static let jointCount = 91
+    /// 91 * 3 * 4 bytes (floats) + 91 validity bytes.
+    public static let byteCount = jointCount * 12 + jointCount
+
+    public var joints: [SIMD3<Float>]
+    public var valid: [Bool]
+
+    public init() {
+        joints = Array(repeating: .zero, count: Self.jointCount)
+        valid = Array(repeating: false, count: Self.jointCount)
+    }
+
+    public func encoded() -> Data {
+        var d = Data(capacity: Self.byteCount)
+        for j in joints {
+            d.appendBE(j.x.bitPattern); d.appendBE(j.y.bitPattern)
+            d.appendBE(j.z.bitPattern)
+        }
+        for v in valid { d.append(v ? 1 : 0) }
+        return d
+    }
+
+    public init?(decoding data: Data) {
+        guard data.count == Self.byteCount else { return nil }
+        let b = [UInt8](data)
+        self.init()
+        var o = 0
+        for i in 0..<Self.jointCount {
+            func f() -> Float {
+                let v = Float(bitPattern:
+                    UInt32(bigEndianBytes: b[o..<o+4]))
+                o += 4; return v
+            }
+            joints[i] = SIMD3(f(), f(), f())
+        }
+        for i in 0..<Self.jointCount { valid[i] = b[o + i] != 0 }
+    }
+}
+
+/// One HEVC access unit + a keyframe flag.
+public struct VideoPayload: Equatable {
+    public var isKeyframe: Bool
+    public var data: Data
+
+    public init(isKeyframe: Bool, data: Data) {
+        self.isKeyframe = isKeyframe; self.data = data
+    }
+
+    public func encoded() -> Data {
+        var d = Data([isKeyframe ? 1 : 0])
+        d.append(data)
+        return d
+    }
+
+    public init?(decoding data: Data) {
+        guard let first = data.first else { return nil }
+        isKeyframe = first != 0
+        self.data = data.dropFirst()
+    }
+}

shared/AVLiveWire/Tests/AVLiveWireTests/WirePayloadsTests.swift

@@ -0,0 +1,25 @@
+import XCTest
+@testable import AVLiveWire
+
+final class WirePayloadsTests: XCTestCase {
+    func testSkeletonRoundTrip() {
+        var f = SkeletonPayload()
+        f.joints[0] = SIMD3(1, 2, 3)
+        f.valid[0] = true
+        f.joints[90] = SIMD3(-4, 5, -6)
+        f.valid[90] = true
+        let decoded = SkeletonPayload(decoding: f.encoded())
+        XCTAssertEqual(decoded, f)
+    }
+
+    func testSkeletonRejectsWrongSize() {
+        XCTAssertNil(SkeletonPayload(decoding: Data([0, 1, 2])))
+    }
+
+    func testVideoPayloadRoundTrip() {
+        let p = VideoPayload(isKeyframe: true,
+                             data: Data([9, 8, 7, 6]))
+        let decoded = VideoPayload(decoding: p.encoded())
+        XCTAssertEqual(decoded, p)
+    }
+}