Replace the dead PartialityModel with a force_still_processing flag

With ScaleOnTheFly now fixed-only and rotation routed through RotationScaleMerge, the
PartialityModel enum carried no information (it was always Rotation for rotation processing,
Fixed otherwise, and mirrored combine_3d). Drop it:

- ScalingSettings: `partiality_mode` (PartialityModel) -> `force_still_processing` (bool);
  SetPartialityModel/GetPartialityModel -> ForceStillProcessing/GetForceStillProcessing.
  Remove the enum.
- DiffractionExperiment: drop GetPartialityModel(); the rotation-vs-stills decision is now
  just GetCombine3D() (set by the tool = rotation && !force_still). The wedge getters no longer
  key off the model (dead since wedge refinement was removed).
- jfjoch_process: `-P/--partiality fixed|rot3d` -> `--force-still-processing` (a rotation
  dataset scaled as independent stills). Auto-detect sets combine_3d for rotation data unless
  the flag is given.
- jfjoch_scale: `-P fixed|rot3d` now toggles combine_3d directly (no PartialityModel).
- jfjoch_viewer: the "process as stills" toggle sets ForceStillProcessing(!rotation_mode) -
  UI unchanged, just wired to the new field.

PartialityModel was never in the OpenAPI, so no generated clients change. Rotation path
behaviour is unchanged (lyso 16.4/99.6%/87.3%); --force-still-processing correctly routes to
ScaleOnTheFly. CUDA + non-CUDA + viewer all build.

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

process/JFJochProcessCommandLine.cpp

@@ -116,8 +116,8 @@ std::string JFJochProcessCommandLine(const ProcessConfig &config,
         if (config.run_scaling) {
             args.emplace_back("-M");
             const auto sc = experiment.GetScalingSettings();
-            if (const auto pm = sc.GetPartialityModel())
-                add("-P", *pm == PartialityModel::Rotation ? "rot3d" : "fixed");
+            if (sc.GetForceStillProcessing())
+                args.emplace_back("--force-still-processing");
             if (!sc.GetMergeFriedel())
                 args.emplace_back("-A");
             if (sc.GetRefineB())