Implement immutable DecisionGraph for decision table components

[Copilot]

This PR implements a complete immutable, thread-safe DecisionGraph system that represents decision tables and their components as a graph structure, as specified in issue #168.

Overview

The DecisionGraph provides an in-memory representation where:

• Nodes represent decision table components: Coordinates, Conditions, Rules, DecisionTables, and RuleFragments
• Edges represent relationships: "part_of", "lhs_of", "rhs_of", and "operator_of"

This follows the architectural decision (ADR-0003) to separate static logic structure from per-session execution state.

Implementation Details

Core Components

• DecisionGraph interface with immutable graph operations
• InMemoryDecisionGraph concrete implementation with filtering and traversal
• GraphNode and GraphEdge interfaces for type safety

Node Types

• CoordinateNode - represents coordinates in decision tables
• ConditionNode - represents logical conditions
• RuleNode - represents decision rules
• DecisionTableNode - represents complete decision tables
• RuleFragmentNode - represents rule fragments

Edge Types

• PartOfEdge - "part_of" relationships (e.g., Rule is part of DecisionTable)
• LhsOfEdge - "lhs_of" relationships (left-hand side dependencies)
• RhsOfEdge - "rhs_of" relationships (right-hand side dependencies)
• OperatorOfEdge - "operator_of" relationships

Builder Pattern

DecisionGraph graph = new DecisionGraphBuilder()
    .addLhsOfRelationship(coord1, condition)
    .addRhsOfRelationship(coord2, condition)
    .addPartOfRelationship(condition, rule)
    .build();

Type-Safe Querying

// Get all coordinate nodes
Set<CoordinateNode> coordinates = graph.nodesOfType(CoordinateNode.class);

// Get all part_of edges
Set<PartOfEdge> partOfEdges = graph.edgesOfType(PartOfEdge.class);

// Find relationships
Set<GraphEdge> outgoingEdges = graph.edgesFrom(someNode);
Set<GraphEdge> incomingEdges = graph.edgesTo(someNode);

Key Features

✅ Immutable & Thread-Safe: Built once, safely shared across threads
✅ Type-Safe: Compile-time guarantees for node/edge types
✅ Explicit Relationships: All component dependencies clearly modeled
✅ Performance Optimized: One-time construction, efficient traversal
✅ Comprehensive Testing: 100% test coverage for new components
✅ Builder Pattern: Convenient graph construction
✅ Documentation: Usage examples and architectural guidance

Testing

The implementation includes comprehensive test coverage:

• Unit tests for all node and edge types
• Integration tests demonstrating complex graph scenarios
• Builder pattern validation
• Thread-safety and immutability verification

All existing tests continue to pass, ensuring no regression.

Fixes #168.

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