CSVEmitter.export_to_csv raised ValueError only in intensity mode;
degenerate exports with --normalize-to-one silently wrote all-zero
CSV files. The guard now runs before the mode split and no file is
written in either mode.
When config.branches carries exactly one branch (e.g. after
--intensity-cutoff filtering), the branch endpoint/transition define
the spectrum, energy grid, analyzer unified branch view and CSV
header. The stale top-level endpoint inherited from the parent decay
no longer leaks into physics (proven wrong on Cs137: spectrum
extended to 1176 keV while the lone survivor ends at 514 keV).
unzhakov