Apple Health exports write ECG CSVs in two shapes. The importer only
understood the paired Lead,Voltage form; the single-column form (Lead
and Unit as metadata rows, a blank separator, then one bare reading per
line) failed every row as malformed and the whole recording was skipped.
The parser now treats a blank line after Lead and Unit metadata as the
start of the waveform, reads one bounded numeric sample per row, and
converts the declared unit to integer microvolts before the existing
amplitude and safe-integer bounds. The paired layout parses exactly as
before, and an unknown unit fails with a fixed message.
Refs #794