Add multi-observer viewshed and line-of-sight profiles (#1145)

[brendancol]

Summary

• New xrspatial/visibility.py module with three functions: cumulative_viewshed, visibility_frequency, and line_of_sight
• cumulative_viewshed counts how many observers can see each cell, calling the existing viewshed() per observer and summing binary masks
• visibility_frequency is cumulative_viewshed / n_observers
• line_of_sight walks a Bresenham transect between two points, returns elevation profile + visibility status, and optionally checks first Fresnel zone clearance (for radio link work)
• 25 tests: correctness, edge cases, dask-numpy parity, coordinate preservation
• Exports added to __init__.py, .xrs accessor, docs reference, README feature matrix
• User guide notebook: 48_Visibility_Analysis.ipynb

Closes #1145

Test plan

• 25/25 tests pass (pytest xrspatial/tests/test_visibility.py)
• Top-level imports work
• Accessor methods work
• Notebook renders correctly