Extract the R-tree into a standalone, prebuilt-wheel package

[cmalinmayor]

spatial_graph's R-tree is currently JIT-compiled at runtime via witty: the first time a
PointRTree/LineRTree is instantiated for a given dtype combination,
RTree.__init__ calls witty.compile_cython, which invokes the system C
compiler and caches the resulting extension module
(_rtree/rtree.py:45-62).

Because motile_tracker depends on tracksdata which depends on spatial_graph,
every end user of motile_tracker needs a working C compiler installed. 😭
This is already causing issues with our Windows-based users.

The rest of spatial_graph JIT-compiles because graph attribute dtypes are
genuinely unknowable until runtime. The R-tree is different: it only ever
stores a node/edge id plus a bounding box, so its compile parameters come from
a small, enumerable set. It can be prebuilt and shipped as a standalone package with precompiled binary wheels so
that common dtype combinations require no compiler and no witty at runtime.

Recommended plan

1. Extract spatial_graph/_rtree/ into a standalone package (rtree-nd).
2. Build-time codegen + cibuildwheel → binary wheels for the
   enumerated combos across all platforms / supported CPythons.
3. Drop the runtime witty fallback by default; optionally expose it behind a
   [jit] extra for strange dtype combinations.
4. spatial_graph depends on rtree-nd; tracksdata can depend on rtree-nd
   directly for BBoxSpatialFilter (it does not need the rest of
   spatial_graph's graph machinery).