v3.2.0

TSR chains plan on the native backend (#184). sscbirrt's TSRs now come from
sstsr's own C++ core (sstsr 3.3), which carries chains, so a chain goal,
start or path constraint no longer falls back to Python. The door demo plans
its chain about 15-20 times faster than on Python.

Migrating from 3.1.0: nothing to change in Python. Install pulls
sstsr>=3.3. C++ consumers of the installed package need sstsr's CMake
package findable (tsr.get_cmake_dir(), from a built wheel; an editable
install has none).

Changed

• sstsr>=3.3,<4 is required, to build and to run. The native TSR is
  sstsr's C++, and sscbirrt's own C++ port of the TSR rules is removed.
• The native backend lowers TSRChain regions. Exact paths match the Python
  bit-for-bit. Elsewhere a chain's distance is sstsr's bounded inverse, an
  upper bound in both backends, held to the same properties rather than to
  the same numbers. A chain whose later link has a non-identity T0_w is
  refused by sstsr 3.3 itself (personalrobotics/sstsr#169).
• The door demo plans only its chain and drops the single-TSR comparison.
  Its README GIF is re-rendered from the native plan.
• The reference artifact gains ur5e_mujoco_tsr_chain_crank: a UR5e turning
  a crank, with start, goal and path constraint one chain. It runs natively
  under auto.

Removed

• C++ consumers: the RPY helpers in sscbirrt/transform.hpp
  (rot_to_rpy, xyzrpy_to_trans and their kin), which only the TSR port used,
  and their sscbirrt._native bindings. sscbirrt/sscbirrt.hpp no longer
  includes the TSR headers; include sscbirrt/tsr/tsr_set.hpp, which needs
  sstsr_cpp (found by find_package(sscbirrt)).