fix: alternate YAML spellings of the version key are invalid, not invisible

Review on this PR: "version": and version : resolve to the same key in
YAML, Psych keeping the last value, so an alternate spelling slipped past
the duplicate counter. One stale exact key plus one alternate-spelled new
value read as a single, stale declaration and the check passed; reproduced
before fixing.

Per the maintainer's direction, the spelling is restricted rather than
parsed, the same answer as the scalar family: a direct-child key that YAML
would read as version under any spelling other than exactly version: is an
explicit invalid telling the author to rewrite it. The set is closed, so
parsing would also be defensible, but the restriction is smaller and keeps
the scanner a scanner.

Three tests for the coexistence shapes, one for the lone alternate
spelling, one pinning that other quoted keys stay untouched. Suite at 34,
green on 3.9 and 3.12.