Steam Frame build, packaging and deployment scripts; render test

- build_scripts/steamframe/: Debian trixie (Qt 6.8) containers for the
  aarch64 cross build, native build and tests; toolchain; scripts to build,
  test (Vulkan validation layers), package (system Qt, ~3 MB) and deploy over
  SSH. Adapted from the vrdashboard template.
- test/render/: render test for the OpenGL and Vulkan backends, driven like
  OverlayController (option OVRAS_BUILD_TESTS).
- RPATH only $ORIGIN, without the build machine's link directories.
- Ignore dist/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/render/TwoTone.qml

@@ -0,0 +1,7 @@
+import QtQuick
+
+// Top half red, bottom half blue: lets the render test check content and orientation.
+Item {
+    Rectangle { width: parent.width; height: parent.height / 2; color: "#ff0000" }
+    Rectangle { y: parent.height / 2; width: parent.width; height: parent.height / 2; color: "#0000ff" }
+}