Support deformable (cloth and soft body) resets in SolverVBD.reset() #3400

Description

SolverVBD.reset() (tracked and added under #3256) resets rigid bodies, joints, and cables per world, but does not reset deformables (cloth and volumetric soft bodies). Particle StateFlags.PARTICLE_Q / StateFlags.PARTICLE_QD are currently ignored, and the method warns that particle resets are unsupported.

This requests first-class masked deformable reset support so cloth and soft-body particles can be restored to their model defaults per world, matching the rigid reset semantics.

Requested behavior

• reset() honors StateFlags.PARTICLE_Q / StateFlags.PARTICLE_QD, copying model.particle_q / model.particle_qd into the state for particles in the worlds selected by world_mask.
• Same masking semantics as the rigid path: world_mask=None also resets global (world == -1) particles; an explicit mask excludes them.
• Works when an external solver integrates the bodies, and for particle-only models.
• Covers both cloth and volumetric (tet) soft bodies through a single path.

Notes on scope

Unlike the rigid side, the particle side needs no solver-history cold-starting: particle_q_prev is rebaselined from the incoming state at the start of every step(), self-contact and body-particle contact buffers are rebuilt per step (no match-index warm-start path for soft contacts), and tet/cloth elasticity is stateless. So the deliverable is the masked state copy plus removing the unsupported-particle warning and documenting the behavior (including regenerating contacts after moving particles, and rebuild_bvh for large displacements with self-contact).

Motivation / Use Case

• Reproducible RL rollouts, sweeps, and regression tests that drop cloth or a deformable and reset to an identical start.
• Completes the VBD reset story begun in Ensure environment resets work correctly with the VBD solver #3256, whose primary deliverable was the rigid/joint/cable reset.

Acceptance criteria

• SolverVBD.reset() resets cloth and soft-body particles per world via PARTICLE_Q / PARTICLE_QD.
• Global-particle vs explicit-mask semantics match the rigid path.
• Tests cover masked restore, flag selectivity, global-particle semantics, and a finite post-reset step for both cloth and tet soft bodies.