A '#' starts a comment only after a space or tab, or at the start of a
line. A '#' adjacent to the previous token is not a comment: 'key: "v"#c',
'[ a ]#c', '[ a,#c', and a '>#c' block-scalar header are malformed, but
the port (like libyaml) accepted them.
Validate the character immediately before the '#' (a plain scalar already
consumes its own trailing blanks, so an adjacent '#' the next-token scan
sees genuinely had no separator). Applied in scan_to_next_token and the
block-scalar header tail.
Surpasses libyaml on 9JBA, CVW2, SU5Z, X4QW; whole-suite conformance
388/402. Canonical event stream stays 308/308.
Mihaela Mihaljevic
2e9975e977