refactor: remove Simple executor implementation

[beinan]

Summary

Removes the legacy Simple executor to simplify the codebase and reduce maintenance burden. All queries now use the DataFusion planner, which provides better performance and more features.

Changes

Deleted (7 files, 724 lines)

• simple_executor/ module (5 files)
• test_simple_executor_pipeline.rs test file

Modified (5 files)

• query.rs: Removed Simple variant, execute_simple() method, and related code (~250 lines)
• lib.rs: Removed module declaration
• graph.rs (Python): Removed Simple from ExecutionStrategy enum
• README.md: Updated documentation to remove Simple references
• graph_execution.rs (benchmarks): Updated to use DataFusion

Breaking Changes

Rust API:

• ❌ Removed ExecutionStrategy::Simple enum variant
• ❌ Removed CypherQuery::execute_simple() method
• ✅ Migration: Use None or Some(ExecutionStrategy::DataFusion)

Python API:

• ❌ Removed ExecutionStrategy.Simple enum value
• ✅ Migration: Use None or strategy=ExecutionStrategy.DataFusion

Behavior

• All queries now use DataFusion planner (same functionality, better optimization)
• No feature loss - DataFusion supports all Cypher features that Simple did
• Performance likely improves due to DataFusion's query optimization

Testing

✅ All 295 unit tests pass
✅ Package builds successfully
✅ No remaining references to Simple executor in codebase

Impact

Users passing ExecutionStrategy::Simple will need to update their code to use the default DataFusion strategy. This is acceptable as DataFusion provides strictly more features and better performance.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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