feat(rldb): heterogeneous u_socket action support

[ElmoPA]

Adds what a U-socket pusher needs alongside the 2-D circle embodiments:

• embodiment: pin PUSHSHAPES_SIM_U_SOCKET = 19. 18 was HUMAN_BIMANUAL under
  the pre-collapse numbering and the existing u_socket datasets/checkpoints
  (the bf__u3 2026-08 lineage) already encode 19.
• transforms: ThetaToRotVec / RotVecToTheta, so an angle is carried as
  (cos, sin) and cannot wrap discontinuously during training; the revert is
  needed because env.step still wants a 3-dim u_socket action.
• pushshapes: get_keymap_with_pusher_cmd + the rotvec transform lists.
• stages_flow: generalise pusher_hold from hardcoded 2-D xy to a
  D-dimensional pose (state[:, :Dd]), with a guard that raises when the state
  carries fewer coordinates than the action, instead of silently padding from
  the wrong columns.
• pl_data_utils: route packed datasets through pack_collate, which emits
  cu_seqlens, rather than the configured collate_fn.

[ElmoPA]

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• feat(rldb): heterogeneous u_socket action support #566 👈 (View in Graphite)
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