[monebzz]

Issue

Fixes #703

LLM Use

As discussed on the issue: an LLM assistant helped draft the code and tests. The changes were reviewed against the YAML spec's c-printable rule and the codebase's own behaviour, and the full test suite passes (3,417 tests, including 4 new ones for this fix).

Description

The parser silently accepted non-printable control characters that the YAML spec excludes. This adds a check during composition, in resolveFlowScalar() and resolveBlockScalar() as suggested on the issue, reporting an UNEXPECTED_TOKEN error for scalar values containing them.

• Double-quoted scalars only flag unescaped control characters — escape sequences like "\u0001" are legal YAML and still parse cleanly
• U+FFFE/U+FFFF are not flagged, matching what the library's own stringifier emits
• Parse side only; the stringify side mentioned on the issue is left as a follow-up