Add the MKV recorder so a node can Record, not only preview

The recorder was the gap: it is FTF's own customised SDK example (a 1139-line
K4A MKV writer), not a stock sample, and it lived only in one machine's build
tree. Bring its source into the repo and build it as part of install.

It writes K4A-format MKV — the format the rest of Ithaca reads — so it stays,
and gets its libk4a from the Orbbec K4A wrapper rather than Microsoft's Azure
Kinect SDK, which is set aside. build_k4a_wrapper.sh builds that wrapper;
build_sdk.sh injects the recorder into the SDK examples tree and builds it there
against the wrapper (it depends on the SDK's own cmake targets). Found by rpath
at runtime, so no LD_LIBRARY_PATH.

install.sh now builds wrapper -> SDK+recorder -> preview server, each reusing an
existing build so the Nano is untouched. If the wrapper is unavailable the
install still completes preview-only and says so.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/install_deps.sh

@@ -17,9 +17,12 @@ echo ">> Dependances pour JetPack ${JETPACK} (L4T ${L4T}, ${SOC})"
 # back to pip when the distro is too old to carry it.
 sudo apt-get update -qq
 sudo apt-get install -y --no-install-recommends \
-    build-essential cmake pkg-config \
+    build-essential cmake pkg-config git \
     libusb-1.0-0-dev libudev-dev liblz4-dev libjpeg-turbo8-dev \
+    libopencv-dev \
     python3 python3-pip
+# libopencv-dev: the MKV recorder links OpenCV (image handling). liblz4/libjpeg:
+# the preview server. libusb/libudev: the SDK and the K4A wrapper.
 
 if ! python3 -c 'import websockets' 2>/dev/null; then
     echo ">> websockets absent — installation via pip"