Adopt basedpyright with a legacy baseline

Limit analysis to the production and deprecation-test scope already checked by
mypy so unrelated tests and fuzz targets do not block adoption.

Pin basedpyright for reproducible diagnostics and baseline the existing
standard-mode findings. This preserves the current code while making new errors
and warnings fail CI. Run the check in one Linux matrix job to avoid duplicating
the same fixed-target analysis across every platform and interpreter.

Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>

Author: Byron