Add per-link articulation physics configuration

[yuecideng]

Description

This PR adds per-link rigid-body physics configuration for articulations. Previously, ArticulationCfg.attrs applied the same RigidBodyAttributesCfg to every link via a single DexSim set_physical_attr call.

Changes

• RigidBodyAttributesOverrideCfg: partial overrides (None = keep base value)
• LinkPhysicsOverrideCfg + ArticulationCfg.link_attrs: named groups with regex link matching (same pattern as joint drive_pros)
• apply_articulation_link_physics: global attrs first, then per-link overrides; errors if a link matches multiple groups
• Articulation.get_link_physical_attr / set_link_physical_attr: runtime API with regex link selection
• ArticulationCfg.from_dict / RobotCfg.from_dict: parse nested link_attrs
• Tests: merge/overlap unit tests + CPU integration tests (TestArticulationLinkPhysicsCPU)
• Docs: sim_articulation.md updated with link_attrs table and example

Example

ArticulationCfg(
    fpath="robot.urdf",
    attrs=RigidBodyAttributesCfg(static_friction=0.5),
    link_attrs={
        "handle": LinkPhysicsOverrideCfg(
            link_names_expr=["handle_xpos"],
            attrs=RigidBodyAttributesOverrideCfg(static_friction=0.87),
        ),
    },
)

Dependencies: none (uses existing DexSim per-link set_physical_attr API).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Screenshots

N/A (simulation physics configuration; no visual UI change).

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Dependencies have been updated, if applicable.

Test plan

• pytest tests/sim/objects/test_articulation.py::TestRigidBodyAttributesOverride
• pytest tests/sim/objects/test_articulation.py::TestArticulationLinkPhysicsCPU

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