[WIP] Correct spatial momentum calculations for fused links.

[mitiguy]

This change is 

[mitiguy]

Updates spatial momentum calculations so we get correct results whether links are fused together or not.

Changes here:

• Changes in MultibodyTree::CalcBodiesSpatialMomentumInWorldAboutWo() and parallel changes to comments and code made in related method MultibodyTree:CalcSpatialInertia() -- whose signature includes std::vector.
• Uses new position and velocity kinematics calculations from PR# 24746.
• New unit tests in fused_welds_test.cc
• Some terminology cleanup to support wider use of "link" internally.

This PR is a separately-reviewable piece of the larger fused-body project issue #24350. There are no user-visible changes here unless fusing is enabled (which is "internal only" for now).

Note: To parallel CalcSpatialInertia(), anticipate a future PR to make a renamed CalcBodiesSpatialMomentumInWorldAboutWo() public into MultibodyPlant.

@mitiguy made 1 comment.
Reviewable status: needs platform reviewer assigned, needs at least two assigned reviewers.