Summary

• Adds a new msgpack-jackson3 submodule (jackson-dataformat-msgpack3) that ports the existing Jackson 2.x integration to Jackson 3.x (tools.jackson namespace)
• Requires Java 17+ and Jackson 3.1.2; the module is excluded from the build on older JDKs so existing Java 8 users are unaffected
• Includes JMH benchmarks (src/jmh/java) runnable via ./sbt "msgpack-jackson3/jmh:run"

Key differences from msgpack-jackson

• Package namespace: com.fasterxml.jackson → tools.jackson
• TokenStreamLocation replaces JsonLocation; byte offset passed as columnNr
• Lightweight MessagePackWriteContext replaces SimpleStreamWriteContext
• MessagePackFactory.snapshot() delegates to copy(); rebuild() implemented via MessagePackFactoryBuilder
• MessagePackSerializedString: all append*, put*, write* methods fully implemented
• NIL map keys now correctly emit PROPERTY_NAME instead of VALUE_NULL
• Generator and parser ThreadLocals use strong references (~8 KB and ~0.2 KB/thread respectively — negligible compared to Jackson's own per-thread buffer retention)

Test plan

• ./sbt "msgpack-jackson3/test" — 112 tests pass
• ./sbt "msgpack-jackson3/jmh:compile" — JMH benchmarks compile cleanly
• Existing msgpack-core and msgpack-jackson tests unaffected