Resolve BNG2.pl from $PATH via $BNGPATH env var #72

Location: bionetgen/core/utils/utils.py:651

Original marker:

# TODO: Figure out how to use the BNG2.pl if it's set
# in the PATH variable. Solution: set os.environ BNGPATH
# and make everything use that route

test_bngexec resolves BNG2.pl from explicit candidate paths today. Users who already manage BNG2.pl via $PATH (the conventional Unix install pattern) still have to set the path explicitly.

What would unblock this: the original author already proposed the path forward — read $BNGPATH from os.environ, and route all BNG2.pl invocations through that. Concretely, add a fallback chain to the resolver: explicit candidate → $BNGPATH → shutil.which("BNG2.pl") → error.

Discovered during: non-Atomizer TODO/FIXME triage sweep, 2026-05-08.