Join existing separate collider objects into one compound mesh collider

[Weisl]

Context

#445 added a creation-time "Join Primitives (J)" toggle that combines multiple colliders into one compound mesh when they're generated together in a single pass (join_primitives, collider_shapes/add_bounding_primitive.py).

This issue is about the retroactive case: a collider was made as a box in one step and a cylinder in a separate step (or at different times), and the user now wants to select both and merge them into a single compound mesh after the fact, without redoing the creation.

Open question

This may already be considered covered in spirit by #445's creation-time join, since the common case (build several primitives for one object in one go) is handled. Flagging explicitly for a scope call: is the retroactive/after-the-fact case worth a dedicated operator, or is #445's toggle sufficient and this should be closed as not planned?

Proposal (if pursued)

New operator (e.g. object.join_colliders_to_compound) that:

• Requires 2+ selected collider objects sharing the same render-mesh parent.
• Reuses bmesh_join (bmesh_operations/mesh_edit.py) for the geometry merge.
• Applies existing collider-renaming logic to the result, consistent with how join_primitives names its output today.

Acceptance criteria

• Selecting 2+ existing collider objects and running the operator produces one compound mesh collider, named per the existing convention.
• Operator validates the selection (rejects non-collider objects, objects with different parents, or fewer than 2 selected) with a clear error message.