Portable Ithaca capture node: one install for every Jetson

Extract the install that was applied by hand to the Jetson Nano into a single
checkout that builds itself for whatever Jetson it lands on.

The two nodes on the rig share no JetPack — tegra210 caps at 4, tegra234 needs
5+ — so nothing binary is portable between them. The source and the procedure
are: install.sh detects the platform (L4T / JetPack / SoC), installs deps,
builds the OrbbecSDK and the preview server from source locally, and generates
a per-user config and systemd unit. The tested bridge.py ships verbatim; all
machine-specific paths live in the generated config, so it stays unmodified —
its one hardcoded path is made home-relative.

The retired GStreamer/RTSP target is dropped: the direct route encodes no video.

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

README.md

@@ -0,0 +1,104 @@
+# Ithaca capture node
+
+The software a Jetson runs to join an Ithaca RGBD capture rig: a Python bridge that
+puts the node on the WebSocket mesh and answers Preview / Record, and a C++ preview
+server that streams each camera's colour (the sensor's own JPEG) and depth (its own
+16-bit millimetres, LZ4-compressed) straight to the Unity viewer.
+
+One checkout, one command, on any Jetson:
+
+```sh
+git clone <this repo> ~/ithaca-node
+cd ~/ithaca-node
+./install.sh
+```
+
+## Why an install script and not a disk image
+
+The rig currently spans two Jetsons with **no JetPack in common**:
+
+| Node | SoC | JetPack | Ubuntu / CUDA |
+|------|-----|---------|---------------|
+| Jetson Nano (2019) | tegra210 | **4.x only** (EOL) | 18.04 / 10.2 |
+| Jetson Orin Nano | tegra234 | **5 or 6** | 22.04 / 12.x |
+
+tegra210 never got a newer L4T and the Orin refuses the old one, so no single OS
+image covers both, and no compiled binary is portable between them — CUDA, glibc
+and the C++ ABI all differ by generation.
+
+What *is* portable is the **source and the procedure**. `install.sh` detects the
+platform and builds the C++ locally against a locally-built OrbbecSDK, so the same
+command produces a correct binary on each machine. Adding a future Orin NX or a Thor
+needs nothing new here — the script compiles against whatever toolchain it finds.
+
+The bridge is pure Python and already runs identically everywhere; keep it that way
+by never giving it a dependency that has to be compiled.
+
+## Layout
+
+```
+install.sh                     one entry point, idempotent
+bridge/
+  bridge.py                    the node client — shipped verbatim, tested in place
+  config.template.json         @PLACEHOLDERS@ filled per user by install.sh
+server/
+  ithaca_rgbd_server.cpp       the preview server
+  CMakeLists.txt               links a prebuilt libOrbbecSDK, no GStreamer
+systemd/
+  ithaca-bridge.service.in     templated with the user and repo path
+  usbfs-memory.service         raises usbfs_memory_mb for the camera bandwidth
+udev/
+  99-obsensor-libusb.rules     reference copy (the SDK's own installer wins if present)
+scripts/
+  detect_platform.sh           L4T / JetPack / SoC, as KEY=value
+  install_deps.sh              apt + websockets
+  build_sdk.sh                 reuse or build OrbbecSDK v2 for this platform
+  build_server.sh              cmake + make the preview server
+```
+
+## What install.sh does
+
+1. Detects L4T / JetPack / SoC (`scripts/detect_platform.sh`).
+2. Installs build and run dependencies (no GStreamer — the direct route encodes no
+   video).
+3. Locates a working OrbbecSDK build, or clones and builds one. A node that already
+   has a good build (the Nano) is left untouched.
+4. Builds `ithaca_rgbd_server` against it.
+5. Generates `bridge/config.json` for the current user — this is where all the
+   machine-specific paths live, so `bridge.py` itself stays unmodified.
+6. Installs the udev rules and the `usbfs-memory` service, and adds the user to the
+   `video` group.
+7. Generates and starts the `ithaca-bridge` systemd service.
+
+Re-run it after a `git pull`: it rebuilds, regenerates the unit and config, and
+restarts the service. It keeps an existing `config.json` (delete it to regenerate)
+and an existing SDK build.
+
+## The SDK is the one thing to watch on a new JetPack
+
+`build_sdk.sh` reuses an existing build when it finds one. On a machine with none it
+clones the official OrbbecSDK v2 and builds it — this is the step most likely to
+need attention on a JetPack it has not been tried on. If it fails, build the SDK by
+hand once and re-run with its location:
+
+```sh
+OB_SDK_ROOT=/path/to/OrbbecSDK_v2 ./install.sh
+```
+
+Override the source with `OB_SDK_REPO` / `OB_SDK_REF` to pin a tag known to build on
+your JetPack.
+
+## Operating the node
+
+```sh
+systemctl status ithaca-bridge      # is it up
+journalctl -u ithaca-bridge -f      # live log
+```
+
+Runtime state the bridge writes beside itself (git-ignored):
+
+- `config.json` — this machine's config (generated).
+- `cam_settings.json` — per-camera settings, kept across restarts.
+- `stream_map.txt` — the live composition: line 1 the cameras to stream, line 2 the
+  alignment (`raw` or `compare`). The preview server re-reads it once a second, so a
+  camera can be turned on or off, and Compare switched, without restarting.