An inline anchor with a tag on the next indented line, above a mapping indented
deeper still (`key: &anchor` / ` !!map` / ` a: b`), left the node introduced by
a further indent token after the properties were consumed. The parser could not
read a node from that indent and rejected it as a missing node.
The indented mapping value now descends through the deeper indent, carrying the
accumulated anchor and tag to the node it introduces, so the properties attach
to the mapping rather than to an empty value.
Fixes yaml-test-suite BU8L; raises parse-level conformance to 312/333 (93.7%)
and value-level to 205/205 (100.0%), with the should-error rate unchanged.