This PR adds JSON encoding and decoding support derived from Schema[A] using DynamicValue as the intermediate representation. #679

Summary

• Introduces a minimal Json ADT for representing JSON values

• Adds JsonEncoder[A] and JsonDecoder[A] typeclasses

• Provides schema-based derivation following the existing pattern:

  Schema[A] ⇄ DynamicValue ⇄ Json

• No Deriver / no reflection / no registers

• Does not modify Schema itself

• Implementation is small, explicit, and consistent with the current codec design

Design Notes

• DynamicValue is the authority for encoding/decoding; Schema drives correctness
• A small structural heuristic is used when converting JSON → DynamicValue (single-field uppercase keys treated as variants).
  This is validated by schema.fromDynamicValue, so incorrect cases fail safely.
• Scope is intentionally limited to codec derivation only (per issue #679)

Tests

Added focused tests covering:

• Simple and nested case class round-trips
• Optional fields
• Failure cases with clear errors
• All tests pass on JVM and JS

Status

• Code formatted (scalafmtCheckAll)
• Full test suite passing
• Ready for maintainer review

/claim #679