Quote scalars with a leading or trailing tab

[gaoflow]

AnalyzeScalar already forces quoting for a scalar with a leading or trailing space, but the whitespace check it uses (buffer.IsSpace()) is space-only, so a leading or trailing tab falls through and the value is emitted as a plain scalar. That output is not self-consistent:

• on read-back the leading/trailing tab is stripped as indentation, so the string round-trips to the wrong value;
• when the tab immediately precedes a flow indicator, the parser rejects the emitter's own output with a SemanticErrorException.

Repro on the default serializer:

var ser = new SerializerBuilder().Build();
var deser = new DeserializerBuilder().Build();

// #732 - throws on the emitter's own output:
var y = ser.Serialize(new List<string> { "\t," });   // "- \t,\n"
deser.Deserialize<List<string>>(y);                   // SemanticErrorException

// silent truncation (a leading space is quoted, a tab is not):
deser.Deserialize<string>(ser.Serialize(" x"));       // " x"  (ok, quoted)
deser.Deserialize<string>(ser.Serialize("\tx"));      // "x"   (leading tab lost)
deser.Deserialize<string>(ser.Serialize("x\t"));      // "x"   (trailing tab lost)

This also covers the tab cases from #493 (e.g. a Dictionary value of "\thello").

Note this is the default emitter path, separate from the \s regex in TypeAssigningEventEmitter that #732 was closed against - that fix only applies with .WithQuotingNecessaryStrings(), so the plain default builder above still reproduces.

Fix: use the existing IsWhite() (space or tab) predicate for the leading/trailing whitespace detection, so a tab is treated like a space. The change is limited to leading/trailing detection - an interior tab is safe unquoted and stays plain ("x\ty" still emits as x\ty and round-trips).

Tests in SerializationTests cover leading/trailing/only tab round-trips, the "\t," list no-throw case, and pin that an interior tab stays plain and that leading/trailing spaces are unchanged. Full suite green on net10.0.