Deduplicate shared logic between _render_shapes and _render_points

Extract 8 helper functions from the near-identical datashader rendering
paths in _render_shapes() and _render_points():

- _apply_datashader_norm: norm vmin/vmax edge-case handling
- _build_datashader_colorbar_mappable: ScalarMappable construction
- _datashader_aggregate: categorical/continuous/no-color aggregation
- _datashader_shade_continuous: continuous color mapping + spread + NaN
- _datashader_shade_categorical: categorical/no-color color mapping
- _render_datashader_result: RGBA image rendering + NaN overlay
- _make_palette: ListedColormap construction
- _decorate_render: legend/colorbar/scalebar decoration

Also refactor the show() dispatch loop in basic.py from 4 if/elif
branches to a table-driven pattern.

No public API changes. No behavioral changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis