Add native Processing SDK

[jshaw]

What changed

• Adds a native Java 17 DataNet client for Processing 4.
• Supports JSON and binary pub/sub, DMX frames, presence, heartbeat, reconnect, subscription replay, and structured errors.
• Dispatches user callbacks on Processing's animation thread.
• Adds beginner, binary/DMX, and presence examples.
• Adds Gradle tests, Javadocs, release packaging, GitHub Pages docs, CI, and tag-driven releases.

Why

DataNet needs a first-class Processing SDK and a release layout accepted by Processing Contributions, rather than asking Processing users to adapt the JavaScript or Python clients.

Validation

• ./gradlew --no-daemon clean buildReleaseArtifacts
• 8 JUnit tests pass.
• DataNet.zip, DataNet.txt, and DataNet.pdex are generated with the required layout.
• All three .pde examples pass Processing 4.5.2 preprocessing and compile against the packaged JAR.
• No existing DataNet name collision was found in the current Processing contributions index.