Add UsKy section (Kentucky, Section ID 26)

[chuff]

Summary

Implements the Kentucky section per the GPP US-States/KY spec, registered as Section ID 26 (client-side API prefix usky).

• Adds UsKy section, UsKyField constants, UsKyCoreSegment, UsKyGpcSegment following the existing UsMn pattern.
• Registers the section in Sections.java and GppModel.java (encode/decode dispatch + getUsKySection() accessor).
• Core subsection has 11 fields including SensitiveDataProcessing (N-Bitfield(2,8)) and KnownChildSensitiveDataConsents. The single MspaMode field replaces MspaOptOutOptionMode + MspaServiceProviderMode, and MspaVersion is the first field.

Test plan

• Added UsKyTest covering encode default, encode all fields, validation rejection, GPC-segment exclusion, round-trip decode, and garbage input
• Updated GppModelTest.testEncodeDefaultAll and testDecodeDefaultsAll to include UsKy
• mvn test on iabgpp-encoder: 370 passing