fix(avlivebody): apply phase 2 review fixes

The phase 2 code review surfaced four follow-ups left after merge.
The joint accessor indexed body.valid without checking its length,
so a short valid array could crash on out-of-range access; it now
guards i < body.valid.count. The viz mode name list was rebuilt on
every modeName() call, so it is hoisted to a static constant. A
Metal shader compile test is added to catch scene.metal regressions
in CI. The dead SceneView placeholder, superseded by LayeredSceneView,
is removed.

avlivebody-mac/Sources/AVLiveBody/SceneUniformBuilder.swift

@@ -22,7 +22,8 @@ enum SceneUniformBuilder {
 
         if let body = skeletons.values.first {
             func j(_ i: Int) -> SIMD3<Float>? {
-                (i < body.joints.count && body.valid[i]) ? body.joints[i] : nil
+                (i < body.joints.count && i < body.valid.count
+                    && body.valid[i]) ? body.joints[i] : nil
             }
             if let hips = j(hipsIdx) {
                 u.body_x = hips.x; u.body_y = hips.y; u.body_z = hips.z

avlivebody-mac/Sources/AVLiveBody/SceneView.swift

@@ -1,15 +0,0 @@
-import RealityKit
-import SwiftUI
-
-/// SwiftUI bridge that hands the SceneController's ARView to the
-/// window and runs `setUp()` once.
-struct SceneView: NSViewRepresentable {
-    let controller: SceneController
-
-    func makeNSView(context: Context) -> ARView {
-        controller.setUp()
-        return controller.arView
-    }
-
-    func updateNSView(_ view: ARView, context: Context) {}
-}

avlivebody-mac/Sources/AVLiveBody/SettingsPanel.swift

@@ -54,10 +54,10 @@ struct SettingsPanel: View {
         .background(.ultraThinMaterial)
     }
 
+    private static let modeNames = ["storm","tunnel","plasma","kaleido",
+        "voronoi","metaballs","starfield","bars","hands3d","openpos"]
     static func modeName(_ i: Int) -> String {
-        let n = ["storm","tunnel","plasma","kaleido","voronoi",
-                 "metaballs","starfield","bars","hands3d","openpos"]
-        return n.indices.contains(i) ? "\(i) \(n[i])" : "\(i)"
+        modeNames.indices.contains(i) ? "\(i) \(modeNames[i])" : "\(i)"
     }
 
     @ViewBuilder

avlivebody-mac/Tests/AVLiveBodyTests/ShaderCompileTests.swift

@@ -0,0 +1,22 @@
+// avlivebody-mac/Tests/AVLiveBodyTests/ShaderCompileTests.swift
+import XCTest
+import Metal
+@testable import AVLiveBody
+
+final class ShaderCompileTests: XCTestCase {
+    func testSceneMetalCompiles() throws {
+        let here = URL(fileURLWithPath: #filePath)   // .../Tests/AVLiveBodyTests/ShaderCompileTests.swift
+        let shader = here
+            .deletingLastPathComponent()             // AVLiveBodyTests
+            .deletingLastPathComponent()             // Tests
+            .deletingLastPathComponent()             // avlivebody-mac
+            .appendingPathComponent("Sources/AVLiveBody/Resources/scene.metal")
+        let src = try String(contentsOf: shader, encoding: .utf8)
+        guard let dev = MTLCreateSystemDefaultDevice() else {
+            throw XCTSkip("no Metal device")
+        }
+        let lib = try dev.makeLibrary(source: src, options: MTLCompileOptions())
+        XCTAssertNotNil(lib.makeFunction(name: "bg_vertex"))
+        XCTAssertNotNil(lib.makeFunction(name: "bg_fragment"))
+    }
+}