feat(avlivebody): render settings state object

Add RenderSettings ObservableObject with Published defaults for
layer visibility, mesh material, video opacity, light intensities,
camera FOV, and panel toggle. Add RenderSettingsTests asserting
all eleven defaults via TDD red-green cycle.

avlivebody-mac/Sources/AVLiveBody/RenderSettings.swift

@@ -0,0 +1,31 @@
+// avlivebody-mac/Sources/AVLiveBody/RenderSettings.swift
+import SwiftUI
+
+/// Live visual settings for AVLiveBody. ContentView observes this and
+/// pushes changes to SceneController. Defaults chosen so the scene is
+/// lit and everything visible out of the box.
+@MainActor
+final class RenderSettings: ObservableObject {
+    // Layer visibility
+    @Published var showSkeleton: Bool = true
+    @Published var showMesh: Bool = true
+    @Published var showVideo: Bool = true
+
+    // Mesh material
+    @Published var meshMetallic: Bool = false
+    @Published var meshRoughness: Double = 0.6
+
+    // Video quad
+    @Published var videoOpacity: Double = 1.0
+
+    // Lights (RealityKit DirectionalLight intensities)
+    @Published var keyIntensity: Double = 4000
+    @Published var fillIntensity: Double = 1500
+    @Published var rimIntensity: Double = 2000
+
+    // Camera
+    @Published var fieldOfView: Double = 55
+
+    // Panel visibility
+    @Published var showPanel: Bool = false
+}

avlivebody-mac/Tests/AVLiveBodyTests/RenderSettingsTests.swift

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+// avlivebody-mac/Tests/AVLiveBodyTests/RenderSettingsTests.swift
+import XCTest
+@testable import AVLiveBody
+
+@MainActor
+final class RenderSettingsTests: XCTestCase {
+    func testDefaults() {
+        let s = RenderSettings()
+        XCTAssertTrue(s.showSkeleton)
+        XCTAssertTrue(s.showMesh)
+        XCTAssertTrue(s.showVideo)
+        XCTAssertFalse(s.meshMetallic)
+        XCTAssertEqual(s.meshRoughness, 0.6, accuracy: 1e-9)
+        XCTAssertEqual(s.videoOpacity, 1.0, accuracy: 1e-9)
+        XCTAssertEqual(s.keyIntensity, 4000, accuracy: 1e-9)
+        XCTAssertEqual(s.fillIntensity, 1500, accuracy: 1e-9)
+        XCTAssertEqual(s.rimIntensity, 2000, accuracy: 1e-9)
+        XCTAssertEqual(s.fieldOfView, 55, accuracy: 1e-9)
+        XCTAssertFalse(s.showPanel)
+    }
+}