Lightweight CRS parser to remove hard pyproj dependency

[brendancol]

Closes #1057

Summary

• Adds a lightweight CRS class that covers the ~150 EPSG codes with Numba fast paths (WGS84, NAD83, NAD27, UTM zones, Web Mercator, Albers, LCC, LAEA, polar/oblique stereographic, CEA, ellipsoidal Mercator)
• _resolve_crs() and _crs_from_wkt() try the built-in table first, fall back to pyproj for anything else
• _grid.py boundary estimation and _source_footprint_in_target use a new transform_points() scatter-point helper that reuses existing Numba kernels, so pyproj.Transformer is only needed for unsupported CRS pairs
• Chunk functions (_reproject_chunk_numpy, _reproject_chunk_cupy) and reproject()/merge() no longer call _require_pyproj() upfront -- pyproj is imported lazily only when the fast path can't handle the CRS

Test plan

• 106 new tests in test_lite_crs.py covering construction, to_dict/to_wkt round-trips, pyproj comparison for all embedded codes, two-tier resolution, scatter-point transforms, grid computation with lite CRS, and monkeypatched no-pyproj integration tests
• 1 new integration test in test_reproject.py for full reproject() with lite CRS
• All 193 tests pass, including the existing 86 reproject regression tests