[TINKERPOP-3261] Enable multiple labels on vertex with configurable label cardinality

[xiazcy]

Introduces configurable label cardinality for graph elements. Vertex cardinality is user-configurable; edge cardinality defaults to ONE and is not yet exposed as a configuration option but uses the same underlying infrastructure.

Vertices can now have zero, one, or many labels controlled by LabelCardinality (defaults to ONE for full backward compatibility). New traversal steps labels(), addLabel(), dropLabel(), and dropLabels() enable label retrieval and mutation. Edge labels remain fixed at cardinality ONE.

Notes on design:

• LabelCardinality is a pure data descriptor enum with min()/max()/mutable fields. Validation logic lives in a separate LabelCardinalityValidator utility.
• mergeV() uses append-only semantics for onMatch labels (no implicit replacement).
• hasLabel('a','b') is OR semantics, use hasLabel('a').hasLabel('b') for AND semantics.
• with("multilabel") is source-level setting for backwards compatibility. Configures elementMap()/valueMap() to return labels as a Set instead of a single String by default.
• Older serialization formats (GraphSON V1/V2/V3, Gryo) silently use label() which returns one non-deterministic label. Only GraphBinary and GraphSON V4 preserve the full label set.

Commits:

1. Core data model, steps, and grammar - LabelCardinality enum, Element.labels(), addLabel()/dropLabels() steps, LabelsStep, LabelsDropVerificationStrategy, MergeVertex multi-label, elementMap()/valueMap() label output via with("multilabel")/with("singlelabel"), GremlinLang propagation, grammar rules, GraphBinary V4 serialization, Java test infrastructure (@MultiLabel/@MultiLabelDefault/@SingleLabelDefault tags)
2. TinkerGraph reference implementation - TinkerVertex multi-label storage (CopyOnWriteArraySet), vertexLabelCardinality configuration property, unit tests
3. GLV implementations (Go, Python, JS, .NET) - GremlinLang with("multilabel")/with("singlelabel") propagation in all GLVs (both strategy-creation and strategy-exists paths), Go With() now variadic (single-arg works), feature test matrix with @MultiLabel/@MultiLabelDefault tags, existing tests use with("singlelabel") for provider safety, Docker server config, Cucumber infrastructure
4. Documentation - Upgrade docs (users + providers), reference docs (elementMap/valueMap/labels/addLabel), TinkerGraph config, provider semantics, for-committers gherkin tags, CHANGELOG

Configuration

To Enable multi-label in TinkerGraph:
gremlin.tinkergraph.vertexLabelCardinality=ZERO_OR_MORE

Testing

• Feature tests (Gherkin): Labels.feature, AddLabel.feature, DropLabel.feature, ElementMap.feature, ValueMap.feature (multi-label matrix)
• Unit tests: LabelMutationStepTest, LabelCardinalityTest, LabelReplacePatternValidationTest, MergeVMultiLabelTest, TinkerVertexMultiLabelTest

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VOTE +1