Add survey area and depth-coverage API

[ijiraq]

Summary

• Expose survey pointing geometry and coverage helpers in Fortran (survey_load, pointing_geom, polygon_area, eta helpers) without relying on n_sur_max in the f90wrap surface
• Add Python SurveyCharacterization / Pointing API and coverage_vs_magnitude() for area and depth coverage
• Add Fortran and Python unit tests for polygon area, pointing geometry, and coverage

Test plan

• make -C F95/tests test_polygon_area
• make -C F95/tests test_pointing_geom
• pytest src/tests/test_survey_coverage.py
• Spot-check coverage results against a known characterization (e.g. CFEPS)

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