fix: isolate calibration temp paths per user (#14619)

* fix: isolate calibration temp paths per user

The calibration temp files under <temp>/calib were file-scope statics
initialized before set_temporary_dir() runs at startup, so they kept
using the shared system temp root and missed the per-user isolation
added in #14607. On Linux every account shares /tmp, so the first user
to calibrate owns /tmp/calib and later users fail to write there, the
same cross-user collision #14607 fixed for model backups, STEP import,
and part skip.

Build the paths lazily from temporary_dir() instead, through a
calib_temp_dir() accessor and a calib_temp_file() join helper. The base
becomes <temp>/orcaslicer_<uid>/calib on Linux and is unchanged on
Windows, where the temp dir is already per user.

Also make StoreParams::path a std::string rather than a non-owning
const char*. The calibration code had to keep a std::string alive
solely to feed that pointer, and the field was uninitialized by
default; owning the string removes the lifetime hazard for all three
callers and makes the entry guard a reliable empty() check. Confined to
the 3mf project exporter (three callers, two internal reads); no
on-disk, format, or ABI impact.

Follows up on #14607 per @Noisyfox's review suggestion.

src/OrcaSlicer.cpp

@@ -7359,7 +7359,7 @@ bool CLI::export_project(Model *model, std::string& path, PlateDataPtrs &partpla
     bool success = false;
 
     StoreParams store_params;
-    store_params.path = path.c_str();
+    store_params.path = path;
     store_params.model = model;
     store_params.plate_data_list = partplate_data;
     store_params.project_presets = project_presets;