Single-quoted scalars share the same flow line folding as double-quoted,
which the previous commit added: a single break folds to a space, a blank
line to one newline, whitespace adjacent to a break is stripped, and the
first-line leading and last-line trailing whitespace are kept. The
single-quoted decoder kept its raw breaks; it now reuses the shared
foldFlowScalarLineBreaks helper, while doubled quotes ('') still escape to
one quote.
Measured against the official YAML test-suite, value-level conformance
rises from 143/169 (84.6%) to 147/169 (87.0%) with no regression.
Parse-level is unchanged: these documents already parsed, just to the
wrong tree.