Add nested multi-resolution volume render backend - #337

Implement efficient Sparse Multi-Scale Grid algorithm (Benitez-Llambay 2025)
for 3D volume rendering of SPH data. Particles are hierarchically assigned
to grid levels based on kernel smoothing length, then scattered onto
coarse-to-fine grids and upsampled back to target resolution. This bounds
per-particle work regardless of smoothing length, providing significant
speedups for simulations with wide kernel distributions.

Features:
- New nested grid backend accessible via backend="nested" parameter
- Adaptive hierarchy with configurable target voxels per kernel (ntarget)
  and number of levels (nlevels)
- Kernel arithmetic matches scatter.py implementation (Wendland-C2)
- Parallel support via Numba prange
- Trilinear upsampling for hierarchy collapse
- Efficient bounding-box tracking to skip empty regions

API additions:
- render_voxel_grid() and render_gas() now accept backend, ntarget, nlevels
- backends dict updated to include "nested" alongside "scatter"
- Added validation and warnings for backend-specific parameters

Tests:
- Added TestNestedVolumeRender class with serial/parallel matching tests
- Tests cover periodic boundary conditions and edge cases
- Comparison against single-resolution backend validates correctness

Documentation:
- Updated volume_render.rst with nested backend description and examples
- Added parameter documentation for ntarget and nlevels

Code quality:
- Passes ruff linter
- Passes numpydoc validation
- All existing tests continue to pass

Refs: Benitez-Llambay A., 2025, MNRAS (in prep)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>