Add smooth Morris interface activity

[svchb]

Summary

• add C1 interface activation for Morris CSF with ColorfieldSurfaceNormal
• combine a tapered color-gradient indicator with a continuous complete-support kernel moment
• accumulate fluid, wall, and rigid-body support moments during the existing normal traversal without another neighbor pass
• weight the curvature stencil continuously by neighboring interface activity and handle vanishing normalization safely
• add configurable taper controls, metadata, documentation, migration notes, and focused regressions

Stack

Depends on #1268, which in turn depends on #1252. 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/ccsf-free-surface-core...rebuild/morris-csf-interface

Scope

The smooth activity applies only to Morris CSF with ColorfieldSurfaceNormal; it does not alter the separate C-CSF geometry. Normal smoothing, shifting/TIC coupling, and a CSS stress-operator rewrite are intentionally excluded.

Verification

• smooth-interface activity tests: 52/52
• rigid/wall support-moment tests: 19/19
• CSS/CSF sphere-normal tests: 199/199
• Akinci sphere-normal tests: 30/30
• rectangular-normal tests: 103/103
• aggregate surface-tension tests: 97/97
• WCSPH system tests: 224/224
• EDAC system tests: 96/96
• rigid-system tests: 182/182
• RHS tests: 1197/1197
• Aqua passed
• documentation build passed
• complete unit target: 22850 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 owns the generic surface-detection and surface_activity pipeline. After #1268 is refreshed, this draft should consume that signal and remove duplicate Morris-owned activity/cache plumbing, retaining only Morris-specific curvature behavior. No additional intermediary stack branch is planned.