rugnux: honour an absolute -o output prefix

configure_offline_output still routed the -o prefix through FilePrefix (the
CheckPath guard), so 'rugnux -o /abs/path' threw 'Path cannot start with slash'
before processing even though the writer path already supported it. Use the
trusted setter here too. Completes the offline absolute-path support (the writer
and Rugnux::Run already skip the guard); the .mtz/.cif and _process.h5 now land
at an absolute -o. The --scale path shares this function and writes via
WriteReflections (no FileWriter), so it is covered as well.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/rugnux_cli.cpp

@@ -378,7 +378,9 @@ std::optional<CrystalLattice> parse_lattice_arg(const char *arg) {
 // path and the full-analysis path (each then sets its own space group and images-per-trigger).
 void configure_offline_output(DiffractionExperiment &experiment, const std::string &output_prefix) {
     experiment.BitDepthImage(32).Compression(CompressionAlgorithm::BSHUF_LZ4);
-    experiment.FilePrefix(output_prefix);
+    // Offline CLI: the operator chose the output path, so allow an absolute -o (the multi-user guard
+    // that FilePrefix() applies is only for remotely-supplied prefixes in the broker/writer).
+    experiment.FilePrefixTrusted(output_prefix);
     experiment.Mode(DetectorMode::Standard);   // full image analysis
     experiment.PixelSigned(true);
     experiment.OverwriteExistingFiles(true);