Witnesses: emit YAML 2.3 violation witnesses #45

SV-COMP 2027 requires YAML violation witnesses, format 2.3+ for Java; GraphML is no longer listed (benchmarks). Correctness witnesses are not expected.

WitnessCreator (repo, v0.1.1) only writes GraphML.

• Emit witness.yml (entry_type: violation_sequence, format_version: "2.3", language: Java, sha256 input_file_hashes, no data_model)
• One function_return waypoint per Verifier.nondet*() call (location = closing ), \result == <literal>), final target at the violating statement
• Columns: the [WITNESS] markers only carry line numbers today
• Release a new WitnessCreator version, bump WITNESS_CREATOR_VERSION/SHA in .github/workflows/gradle.yml
• Validate locally (Local witness validation: use pinned SV-COMP 2027 validators #47)

Refs: Java guide, schema, example examples/java/ExDarko_false.witness-2.3.yml. Watch sv-witnesses !120 (2.4 changes function_return to ext_java_expression).