Add xdsgui module: prebuilt binary + generate_XDS.INP from XDS wiki

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

xdsgui/README.md

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+# xdsgui
+
+Graphical user interface for XDS data processing: visualizes control
+images and statistics, helps with writing XDS.INP.
+
+- Upstream: https://wiki.uni-konstanz.de/xds/index.php/XDSGUI
+- License: free of charge (see upstream)
+
+## How it is built
+
+Upstream ships a prebuilt, unversioned Linux binary at a fixed URL, so
+the module version is the download date. `pbuild::install` wgets
+`xdsgui` and the `generate_XDS.INP` helper script from
+https://wiki.uni-konstanz.de/pub/linux_bin/ into `$PREFIX/bin` and
+marks them executable.
+
+Loading the module puts `$PREFIX/bin` on `PATH` and auto-loads the
+runtime dependencies `H5ToXds/1.1.0`, `xds/20260616` and
+`autoPROC/20240710` (xdsgui calls xds and the helper tools).
+
+## Verifying the install
+
+```sh
+module use MX unstable && module load xdsgui/<version>
+which xdsgui generate_XDS.INP   # both under .../xdsgui/<version>/bin
+module list                     # H5ToXds, xds, autoPROC pulled in
+xdsgui                          # on a node with X forwarding; GUI opens
+```
+
+## Adding a new version
+
+Add the new download date under `versions:` in `files/config.yaml`,
+then on a Ra login node: `modbuild build <version>` (modbuild/2.1.2).