Add balanced CSS surface tension

[svchb]

Summary

• rebuild the balanced continuum-surface-stress (CSS) portion of the Surface Tension Prototype V1  #1253 prototype as a focused layer
• evaluate delta_s * (I - n * nᵀ) directly in the symmetric fluid pair interaction instead of caching a per-particle stress tensor and subtracting a global maximum
• reuse the C1 color-gradient and complete-support activity introduced by Add smooth Morris interface activity #1269 for SurfaceTensionMomentumMorris
• accumulate the continuous support moment during existing fluid and boundary normal traversals and apply the symmetric correction 2 / (q_a + q_b)
• retain exact pairwise linear-momentum conservation for constant smoothing length, including unequal particle masses, densities, and interface activities
• preserve VTK stress diagnostics by constructing the physical tensor only when writing output
• document the balanced operator, migration of ideal_density_threshold, and one-phase surface-delta normalization

Stack

Depends on #1269, which depends on #1268 and #1252. This is the correct branch point for the CSS substack because #1269 provides the shared smooth interface activity and support-moment plumbing. It intentionally does not depend on #1270, whose C-CSF planar boundary-integral geometry is unrelated; #1270 and this PR are sibling layers.

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/morris-csf-interface...rebuild/balanced-css

Scope

This first CSS layer covers one-phase colorfield geometry, smooth interface activity, the balanced fluid-fluid stress divergence, and wall/rigid support completion. Wetted-area/contact-line models, normal smoothing, interface-aware shifting/TIC, and other later CSS prototype features remain outside this PR. C-CSF boundary-integral geometry remains isolated in #1270.

Verification

• aggregate fluid tests: all passed
• surface-tension tests: 116/116, including balanced CSS 21/21 and static Laplace balance 4/4
• CSS flat-pool wall geometry: 6/6
• RHS conservation tests: 1197/1197
• WCSPH system tests: 224/224
• EDAC system tests: 96/96
• VTK tests: 24/24 plus focused CSS tensor diagnostics
• Aqua passed
• documentation build passed
• complete unit target: 22875 passed; only the established baseline set remained (1 failed, 3 errored)
• JuliaFormatter 2.1.1 and git diff --check passed

[svchb]

Dependency note: #1282 now provides generic surface_activity. When this draft is rebased, it should remove duplicate model-owned activity caches and consume the shared signal, while keeping only CSS-specific support normalization needed by the balanced operator. No additional intermediary stack branch is planned.