Add Ion codec for kyo-schema

[hearnadam]

Summary

• Adds Ion encode/decode entry points plus IonReader and IonWriter.
• Supports schema-shaped Ion text: structs, lists, maps, blobs, typed nulls, comments, symbols as strings, timestamps, long strings, special floats, and annotations as metadata.
• Vendors selected Apache-2.0 amazon-ion/ion-tests resources under kyo-schema shared test resources with provenance, license, and corpus coverage.
• Documents that Ion annotations are accepted and validated as input syntax, but are not preserved by schema decoding or emitted by encoding.

Tests

• sbt 'kyo-schema/testOnly kyo.IonTest kyo.IonCorpusTest'
• sbt 'kyo-schema/test'

Notes

• Ion.decode is a single-value schema decode API. Tests that use upstream Ion stream files assert selected root values explicitly rather than treating streams as schema lists.