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>

build_scripts/steamframe/container-build.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# Builds OVRAS for the Steam Frame inside a Debian container (Qt 6.8, the
+# series of the Frame's system Qt). Result: build/steamframe-<mode>/Release/,
+# packaged by package-release.sh. Adapted from the vrdashboard template.
+#
+#   build_scripts/steamframe/container-build.sh cross   x86_64 host -> aarch64
+#   build_scripts/steamframe/container-build.sh native  for the host's own
+#                                     architecture (on the Frame: native arm64)
+#   build_scripts/steamframe/container-build.sh <mode> shell
+#                                     interactive shell in that container
+#
+# Uses podman if available, else docker (override with CONTAINER_ENGINE).
+set -euo pipefail
+
+here="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+root="$(cd "$here/../.." && pwd)"
+mode="${1:-}"
+case "$mode" in
+    cross)  dockerfile=cross-arm64.Dockerfile
+            toolchain="-DCMAKE_TOOLCHAIN_FILE=build_scripts/steamframe/aarch64-linux-gnu.cmake" ;;
+    native) dockerfile=native.Dockerfile; toolchain="" ;;
+    *) echo "usage: $0 cross|native [shell]" >&2; exit 2 ;;
+esac
+image="ovras-steamframe-${dockerfile%.Dockerfile}"
+engine="${CONTAINER_ENGINE:-$(command -v podman || command -v docker)}"
+build="build/steamframe-$mode"
+
+"$engine" build -q -t "$image" -f "$here/$dockerfile" "$here" >/dev/null
+
+# Run as the calling user, so the build tree isn't owned by root.
+run=("$engine" run --rm -v "$root:$root:z" -w "$root")
+if [[ "$(basename "$engine")" == docker ]]; then
+    run+=(--user "$(id -u):$(id -g)")
+else
+    run+=(--userns=keep-id)
+fi
+[[ -t 0 ]] && run+=(-it)
+
+if [[ "${2:-}" == shell ]]; then
+    exec "${run[@]}" "$image" bash
+fi
+
+# OVRAS_STEAM_FRAME: Frame capabilities and defaults
+# (src/utils/platform_capabilities.h).
+"${run[@]}" "$image" bash -c "
+    set -e
+    cmake -S . -B $build -G Ninja -DCMAKE_BUILD_TYPE=Release \
+          -DOVRAS_STEAM_FRAME=ON $toolchain
+    cmake --build $build
+"
+echo "Built: $root/$build/Release"