Add free-surface tangential shifting

[svchb]

Summary

• rebuild the opt-in free-surface shifting layer from the Surface Tension Prototype V1  #1253 prototype on top of normal smoothing and diagnostics Add normal smoothing and VTK diagnostics #1272
• add exported FreeSurfaceTangentialShifting support to callback-based Sun (2017) and consistent Sun (2019) particle shifting
• use the smooth interface activity to progressively remove the component along the raw surface normal while retaining full interior shifting
• clamp invalid activity ranges and preserve the original shifting velocity for non-finite activity or undefined normals
• validate the treatment for Morris CSF/CSS colorfield geometry and Morris C-CSF geometry in WCSPH and EDAC systems
• persist the selected treatment in system metadata and document the formulation, supported combinations, and limitations

Stack

Depends directly on #1272. #1272 depends on #1271, #1269, #1268, and #1252. C-CSF planar boundary geometry #1270 remains a sibling substack and is not required here.

Fork branches cannot be selected as bases for pull requests into the upstream repository, so this PR targets main and temporarily shows the preceding commits.

Review-only diff: svchb/TrixiParticles.jlOpen@rebuild/css-normal-smoothing...rebuild/free-surface-tangential-shifting

Scope

Tangential shifting is an explicit regularization option. It does not change raw or smoothed capillary geometry, interface activity, surface-delta magnitude, surface-tension forces, TVF, wetting/contact models, or tensile-instability control. The default free_surface_treatment=nothing path retains the existing closed-system behavior.

Verification

• shifting-technique tests: 52/52
• WCSPH system tests: 224/224
• EDAC system tests: 96/96
• RHS conservation tests: 1197/1197
• aggregate fluid suite passed
• disabled behavior, transition blending, non-unit and zero normals, invalid values, raw-versus-smoothed normal selection, constructor validation, and integrated treated-versus-untreated shifting passed
• Aqua passed
• documentation build and doctests passed
• complete unit target: 22950 passed; only the established baseline set remained (1 failed, 3 errored)
• JuliaFormatter 2.1.1 and git diff --check passed

[svchb]

Dependency note: #1282 makes surface_activity a generic surface-method output. On rebase, tangential shifting should consume that signal and the normal capability directly instead of depending on Morris-specific activity plumbing. No additional intermediary stack branch is planned.