Add tests for PAM sidecar reader edges and symbology dask stats

Test-coverage sweep of the geotiff subpackage; test-only, no source changes.

read_pam_sidecar edges (new tests/unit/test_pam_reader_edges.py): the
round-trip write tests always emit a full thematic RAT, so four fall-through
paths in _parse_rat / read_pam_sidecar were never hit: a RAT with no Name
column returns {}, a named RAT with zero rows falls back to CategoryNames,
CategoryNames with no RAT at all, and a FieldDefn missing its Usage element
is skipped. This takes _pam.py to full branch coverage.

_symbology dask stats (added to write/test_symbology_sidecar_3537.py):
_dask_finite_stats had only run with nodata=None, so the sentinel-exclusion
where-mask, the dask all-NaN count==0 -> None branch, and StreamingStats over
integer buffers (with and without a sentinel) were untested. The dask+cupy
leg ran and passed on a CUDA host.

Also adds the branch_cov column to the sweep state CSV and records the
geotiff row.

Author: brendancol