A node with an explicit tag is not subject to implicit plain-scalar
resolution, but composeScalarValue fell through to it whenever the tag was not
a recognized core type. So a local tag (`!foo`), the non-specific `!`, or an
unknown verbatim URI still let `! 12` resolve to the integer 12 and
`!Thing false` to a boolean.
When a tag is present but not a core type, fix the node as the string of its
content instead of resolving it. Recognized core tags and untagged scalars
are unchanged. Two compatibility tests asserted the old behavior (a custom-
tagged `true` as a boolean); they are corrected to expect the string, the
behavior the yaml-test-suite case S4JQ requires.
Raises yaml-test-suite value-level conformance to 188/194 (96.9%) with the
parse-level and should-error rejection rates unchanged. 452 tests pass.