Distinguish absent vs empty required field in YAML gate

[VPDPersonal]

Summary

• The required-field gate's pure-YAML scene scan (SerializeReferenceYamlEditor.FindUnsetRequiredFields) no longer treats a totally absent top-level field key as a violation. Unity omits a serialized field from YAML when the object was last saved before the [TypeSelector(Required = true)] field was added (and for stripped / nested-prefab docs), so the old behavior produced a CI false-positive (-srGateRequired at Fail severity exited 1 on a project that is valid once reopened).
• Both field helpers now return a tri-state FieldState (Absent / PresentSet / PresentUnset); only PresentUnset is reported. An absent key is left for a reserialize, which is exactly how the object-load path (which can read defaulted properties) already behaves.
• Added an absent-keys YAML fixture and a regression test asserting no violations are reported when the required keys are missing from the document.

Notes for review

• The object-load path (SerializeReferenceRequiredGate.IsViolation / CollectRequiredViolations) is intentionally unchanged: it walks live SerializedObject properties, where every declared field is present (Unity reserializes on load), so it has no absent-key false-positive. The asymmetry is the point — the YAML scan can't reserialize, so it must not flag what it can't see.
• The two helper methods keep their Is…Unset names despite now returning a tri-state enum; they have no other callers, so renaming would be churn outside this finding's scope.

Linked issues

Refs #49 - addresses review finding #49 (comment)