Add C-CSF free-surface core

[svchb]

Summary

• add CorrectedCSFSurfaceNormal, an explicit free-surface implementation of the corrected continuous-surface-force interface geometry from Vergnaud et al. (2022)
• compute normals from the renormalized minimum-eigenvalue gradient, curvature from the corrected divergence and thin-jet filter, and the surface delta with Shepard correction
• correct SurfaceTensionMorris to apply its local CSF acceleration once per particle while retaining the one-phase surface delta
• wire the local Morris acceleration into WCSPH and EDAC and validate the supported model combination
• add focused 2D/3D regressions, metadata, documentation, migration notes, and the Vergnaud citation

Stack

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@bug_fixed...rebuild/ccsf-free-surface-core

Scope

This implements the single-fluid free-surface C-CSF core. Boundary-integral and contact-angle terms are intentionally not included.

Verification

• C-CSF 2D geometry tests: 11/11
• C-CSF 3D curvature tests: 4/4
• surface-tension tests: 37/37
• WCSPH system tests: 224/224
• EDAC system tests: 96/96
• RHS tests: 1197/1197
• Aqua passed
• documentation build passed
• complete unit target: 22814 passed; only the established baseline set remained (1 failed, 3 errored)
• JuliaFormatter 2.1.1 and git diff --check passed

[svchb]

Dependency note: #1282 is now the shared surface-method foundation. Before this draft advances, it should be rebased onto #1282 and CorrectedCSFSurfaceNormal should follow the new AbstractSurfaceNormalMethod / surface_method contract while providing the generic surface_activity signal. No additional intermediary stack branch is planned.