/claim #682

Fix: MessagePack record decode is now forward-compatible:

• ignores unknown fields (unpacker.skipValue)
• order-independent (map semantics)
• precomputed field offsets for decode

Adds regression test: record can decode out-of-order fields and ignore unknown fields.

Local verification:

• sbt “schema-messagepack/scalafmt” “schema-messagepack/scalafmtCheck” “schema-messagepack/test”