An explicit block scalar indentation indicator is the content indentation
relative to the block scalar's parent node. For an inline mapping value
(`- aaa: |2`) the parent is the mapping, but a compact inline mapping never
pushes its indentation onto the main stack, so the indicator was added to the
stack level (0) rather than the mapping's (2). The content indentation then
came out too small and the next mapping key was pulled into the scalar.
Capture at the header whether the block scalar is an inline mapping value
(a mapping value was seen on its line). When it is, fold the deepest
applicable implicit indentation into the indicator's base; a direct sequence
item (`- |1`), which has no mapping value on its line, keeps the sequence's
own level, so the explicit indicators on sequence items stay correct.
Raises yaml-test-suite value-level conformance to 191/194 (98.5%) with the
parse-level and should-error rejection rates unchanged. 463 tests pass.