[FEATURE] Add 'get_weld_constraints' API.

[LeonLiu4]

Adds RigidSolver.get_weld_constraints() – returns an (N, 3) NumPy array (env_id, link1_idx, link2_idx) of all active weld constraints, enabling quick inspection and test assertions without poking internal Taichi fields.

Resolves #1365

[duburcqa]

Could you take inspiration of get_contacts? i.e. return a dict instead of a raw tensor, and fetch all relevent taichi field values in a single kernel call.

[davidhommie]

Too much additions leads to error

[YilingQiao]

@LeonLiu4 can you merge the latest main? You might need to modify the kernel a little bit to adapt to the new struct of array access pattern

[YilingQiao]

@davidhommie can you provide a script? Or a link to the issue you mentioned?

[MRiabov]

Folks, maybe you would merge a basic version of this? It's been a month since #1365. I currently have to write strange logic to assert weld constraints existence in my tests, e.g. moving a part by an arbitrary distance, and asserting that welded parts have to be equally moved.

[duburcqa]

@MRiabov I understand your frustration but we only merge features when it is ready, and we were busy with other features during the last few weeks.