Fix serialization of strings starting with YAML indicator characters

[rioki]

This PR corrects a round-trip issue where certain string values were emitted as plain scalars even though YAML interprets them as structural tokens. For example, a value like "***" was serialized as:

pass: ***

which YAML parses as an alias, causing deserialization to fail.

The serializer now performs a conservative scan of escaped string contents and quotes strings that begin with, or contain, YAML indicator characters that could change parsing semantics (e.g. *, &, !, #, [/], {/}, etc.). This preserves the intended string type without modifying the parser or scalar_scanner.

All existing tests pass, and the added round-trip test covering this case succeeds.

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• The test suite compiles and runs without any error.
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[rioki]

Not sure why the CI tripped... it failed on git checkout... If I can do anything to help on my end.