rename: Path -> Polyline, avoids clashing with pathlib.Path

[petercorke]

Summary

• Path is renamed to Polyline -- it's a heavily overloaded name
  (pathlib.Path, matplotlib.path.Path, os.path) and RTB in particular
  imports pathlib.Path pervasively, so any RTB code that also wants this
  shape needs an import alias just to avoid shadowing. The class's own
  docstring already called itself "a polyline through a sequence of
  waypoints", so Polyline is the more accurate name too, not just a
  workaround.
• Path has been in a released version since 1.3.0 (currently on PyPI),
  so it's kept as a deprecated alias rather than removed outright -- same
  pattern as Box -> Cuboid: a subclass that raises FutureWarning in
  __init__ then delegates to super().
• The wire-protocol string (stype="path") is left unchanged -- it's an
  internal renderer identifier, not the Python API surface causing the
  clash, and touching it would need a synchronized change on the Swift
  side for no real benefit.
• Docs (api.rst) and tests updated to match; Polyline gets the primary
  entry, Path still gets its own (deprecated) autoclass entry, same
  treatment as Box gets today.

Test plan

• pytest tests/test_Shape.py tests/test_collision.py -- 170 passed
• New test_Path_warns_and_is_a_polyline covers the deprecation:
  warns FutureWarning, isinstance(x, Polyline) holds, behaviour
  unchanged
• repr() correctly distinguishes Path(...) from Polyline(...),
  matching existing Box/Cuboid behaviour
• docs && make html -- clean, no new warnings introduced (33
  pre-existing, unrelated SceneNode duplicate-description warnings)
• Grepped Swift and RTB for direct spatialgeometry.Path/gm.Path
  usage -- none found, so nothing external breaks even without the
  deprecation shim; the shim covers any WIP code not yet checked in

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