Deprecate dual implementations, add integration tests

[stevei101]

Summary

• Deprecate orphaned generic Node<S>, Edge<S>, Router<S> traits (unused, will be removed in v0.3.0)
• Add 9 integration tests for complex graph patterns (multi-branch routing, cycles, error propagation, 50-node deep chains, concurrent state, conditional edges, finish semantics, messages)
• Document architectural decision in ADR-001 and migration guide
• Add CHANGELOG.md

Context

The codebase had dual implementations creating technical debt:

• Active: NodeExecutor + AgentState + GraphRunner (used by all examples and built-in nodes)
• Orphaned: Generic Node<S>, Edge<S>, Runtime<S> (never wired into lib.rs, runtime.rs won't even compile against current CompiledGraph)

This PR re-adds node and edge modules with #[deprecated] attributes, marks runtime.rs as dead code, and adds comprehensive test coverage for the active implementation. See ADR-001 for rationale.

Related: #2 (Epic 1 confirmed complete)

Test plan

• cargo test — 56 tests pass (47 unit + 9 new integration)
• cargo clippy — warnings only in deprecated code
• Integration tests cover: multi-branch conditional routing, cycle exit conditions, recursion limits, error propagation, 50-node deep linear graph, concurrent state mutation, conditional edge routing, finish semantics, message manipulation

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