Tests: four failures on Windows (autosave paths, GPU viewer adapter, audio preview race)

- automation / cli autosave restart tests: the MCP instructions embed the
  auto-save state as JSON, so a Windows path's backslashes are escaped there;
  look for the JSON form of the path.
- ui_gpu: egui_kittest prefers a CPU adapter, which on Windows is WARP on
  DirectX 12, where FXC fails to compile the compositor's `pointwise` kernel
  ("compilation aborted unexpectedly"), so the viewer stayed on the CPU and
  the test failed before comparing anything. Build the harness on the adapter
  the headless compositor picked (falling back to egui_kittest's choice).
- ui_audio_scrub (#103 test): with a fixed 2 s comp, a machine with many
  render threads cached the whole preview before the first step, so the sound
  rightly started at once and no silent frame was counted (3 of 24 runs
  failed with 8 copies in parallel). Size the comp to four times the frames
  playback requests per step, so caching takes at least four steps (0 of 24).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/effectcraft-cli/tests/cli.rs

@@ -373,7 +373,9 @@ fn mcp_autosave_survives_kill_and_restart_without_touching_desktop_settings() {
     assert_eq!(manifest["ended"], false);
     let mut restarted = McpChild::start(&config, true);
     let init = restarted.rpc("initialize", json!({}));
-    assert!(init["instructions"].as_str().unwrap().contains(path));
+    // The instructions carry the auto-save state as JSON, where a Windows path's backslashes
+    // are escaped.
+    assert!(init["instructions"].as_str().unwrap().contains(&Value::from(path).to_string()));
     assert_eq!(init["_meta"]["effectcraftAutoSave"]["previousSessions"][0]["autosave"], path);
     restarted.tool("open_project", json!({"path":path}));
     let comp = restarted.tool("get_comp", json!({"comp":"Survives restart"}));