Bring RamAirKite.jl onto SymbolicAWEModels v0.18: Delete the duplicated inertia integral from VortexStepMethod

[1-Bort-1]

center_to_com!, calculate_inertia_tensor and calc_inertia_y_rotation live in
src/obj_adapter/obj_geometry.jl (:246, :302, :341) and are exported from
ObjAdapter.jl:48. SymbolicAWEModels carries its own copies
(center_of_mass and calculate_inertia_tensor in src/obj_adapter.jl,
calc_inertia_y_rotation in src/rigid_body.jl:191) and uses only
ObjAdapter.read_faces from here. Two implementations of one inertia integral in
two packages means nothing will ever report that they diverged.

Delete the three functions, drop them from the export list, delete their tests in
test/ram_geometry/test_kite_geometry.jl and test/obj_adapter/test_obj_adapter.jl,
and remove them from docs/src/private_functions.md:247-248. read_faces stays.
Nothing inside VortexStepMethod calls them, and a code search of OpenSourceAWE and
1-Bart-1 (RamAirKites, V3Kite, BeyondTheSim, SymbolicAWEModels) found no importer
on 2026-09-16; repeat that search before deleting. Removing public exports is
breaking: BREAKING: entry in the changelog.

Unit vsm-delete of plan plans/ramairkite-obj-yaml-migration.md, tracked in 1-Bart-1/Agents#401.
https://github.com/1-Bart-1/Agents/blob/eed96c809195812611065fddac64130fd4d419c9/plans/ramairkite-obj-yaml-migration.md

[1-Bort-1]

The plan changed what this unit asks for, so the body above is now vsm-delete as plans/ramairkite-obj-yaml-migration.md words it today. Read it again before you go on.

https://github.com/1-Bart-1/Agents/blob/eed96c809195812611065fddac64130fd4d419c9/plans/ramairkite-obj-yaml-migration.md

[1-Bort-1]

Per review: restoring center_to_com!, calculate_inertia_tensor and calc_inertia_y_rotation with their export, tests and docs, each now emitting a deprecation warning pointing at SymbolicAWEModels; the changelog entry becomes a deprecation, not BREAKING. Builds on #325's branch. Removal waits for the next breaking release.