Add interface-aware tensile instability control

[svchb]

Summary

• rebuild the opt-in interface-aware tensile-instability layer from the Surface Tension Prototype V1  #1253 prototype on top of free-surface tangential shifting Add free-surface tangential shifting #1273
• add exported InterfaceAwareTensileInstabilityControl(strength=...) for WCSPH and EDAC systems
• apply TIC in fully supported fluid interiors and blend back to conservative continuity-density pressure using the maximum pair interface activity
• preserve conservative pressure for fluid-boundary pairs, unsupported fluid neighbors, non-finite activity, and TVF tensor terms
• validate ContinuityDensity, unclipped WCSPH pressure, supported Morris CSF/CSS or Morris C-CSF geometry, and the absence of asymmetric kernel-gradient corrections
• persist the selected policy and strength in system metadata and document the formulation and supported combinations

Stack

Depends directly on #1273. #1273 depends on #1272, #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/free-surface-tangential-shifting...rebuild/interface-aware-tic

Scope

This layer only gates the existing Sun TIC pressure correction with interface activity. It does not change interface detection, raw or smoothed normals, surface-delta magnitude, surface-tension forces, particle shifting, TVF behavior, wetting/contact models, or the legacy tensile_instability_control path. No tensile-stability validation study from the prototype is included.

Verification

• pressure-acceleration tests: 53/53, including 44 interface-aware checks
• WCSPH system tests: 224/224
• EDAC system tests: 96/96
• RHS conservation tests: 1197/1197
• aggregate fluid suite passed
• strength bounds, pair-activity blending, activity clamping, non-finite fallback, fluid-boundary fallback, unsupported-fluid fallback, TVF tensor fallback, constructor validation, C-CSF support, and metadata output passed
• Aqua passed
• documentation build and doctests passed
• complete unit target: 22994 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, interface-aware TIC should consume that shared signal directly and drop its dependency on Morris-specific activity plumbing. No additional intermediary stack branch is planned.