A plain scalar inside a flow collection whose next line begins with a flow indicator (',' ']' '}') or the ':' value indicator was retaining the folded line break as a trailing space, so [\nfoo: bar\n] produced the value 'bar ' instead of 'bar'. YAML 1.2 section 7.3.3 strips trailing whitespace from a plain scalar; end the scalar at such a line without folding the break.
Derived from the plain-scalar production, not pattern-matched per case, and verified against the canonical event gate (exact test.event comparison): fixes 4FJ6, 4MUZ/02, CT4Q, LP6E, NKF9, QF4Y, VJP3/01, Y79Y/002, ZF4X, and ZK9H with zero regressions. Exact canonical-event conformance rises from 254 to 264 of 308 valid cases. Adds a gated triage diagnostic to the event gate.