Add RRIM, Multi-Hillshade RGB, and Local Dominance visualizations

Three new visualizations complementing existing SVF/openness/LRM/MSRM:

- RRIM (Red Relief Image Map): RGB composite combining positive openness
  (R), inverted slope (G), negative openness (B). Uses ray-tracing
  to compute both openness values in a single pass.

- Multi-Hillshade RGB: 3 azimuths (315°, 135°, 45°) mapped to R/G/B
  channels with slope blending. Color reveals structure orientation.

- Local Dominance: (dem - local_min) / (local_max - local_min) using
  min/max filters. Measures local height position — complements openness.

Also adds:
- _compute_openness_both() helper for shared ray-tracing (used by RRIM)
- xp_maximum_filter() in gpu.py (GPU/CPU abstraction)
- Entries in COLORMAPS, RGB_LEGENDS, VIZ_STEPS, and is_rgb detection
- All NaN handling follows existing patterns (nan_mask restoration)

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

CLAUDE.md

@@ -4,7 +4,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 ## Project Overview
 
-LiDAR archaeological processing pipeline that generates 17 terrain visualizations from LAZ/LAS point clouds. Runs in Docker with optional NVIDIA GPU acceleration (CuPy). Designed for French LiDAR HD data in Lambert 93 (EPSG:2154).
+LiDAR archaeological processing pipeline that generates 20 terrain visualizations from LAZ/LAS point clouds. Runs in Docker with optional NVIDIA GPU acceleration (CuPy). Designed for French LiDAR HD data in Lambert 93 (EPSG:2154).
 
 ## Commands