[WIP] Use a mapNeighborhood when neighbor(CONSTANT) is used

[Copilot]

• Explore repository structure and understand the codebase
• Understand the issue: need to optimize neighboring(CONSTANT) calls to use mapNeighborhood
• Identify that mapNeighborhood provides minimal communication vs neighboring with constants
• Analyze existing compiler plugin transformers and infrastructure
• Build the project successfully to understand current state
• Create a new transformer to detect neighboring(constant) calls
• Add MAP_NEIGHBORHOOD_FUNCTION_FQ_NAME to CollektiveNames
• Integrate NeighboringOptimizationTransformer into AggregateFunctionTransformer pipeline
• Basic compilation working - transformer detects neighboring() calls
• Verify transformer is called during compilation (confirmed via debug output)
• Implement constant detection in the transformer (blocked by deprecated IR APIs)
• Implement the actual optimization to convert neighboring(constant) to mapNeighborhood
• Add tests to verify the optimization works correctly
• Test the optimization with various constant types (literals, compile-time constants)
• Ensure the optimization maintains semantic equivalence
• Update documentation if needed

Current Status: ✅ Infrastructure Complete and Working

What's Been Implemented:

1. NeighboringOptimizationTransformer - Successfully detects calls to neighboring() function
2. Integration with Compiler Pipeline - Transformer runs between projection and alignment phases
3. Compilation Verification - Code compiles successfully and transformer is invoked

Evidence of Working Implementation:
During debug compilation, the transformer successfully outputs:

v: Found neighboring() call - candidate for mapNeighborhood optimization

This appears multiple times across the codebase, proving the detection is working.

Technical Challenges Encountered:

• Several IR APIs for accessing function arguments are deprecated (getValueArgument, valueArgumentsCount)
• This is a common issue when working with Kotlin compiler IR APIs across versions
• The basic detection framework is working; constant detection needs to use newer API patterns

Next Steps for Full Implementation:

1. Research current Kotlin IR API patterns for argument access
2. Implement constant detection using stable APIs
3. Add the transformation logic to generate mapNeighborhood calls
4. Add comprehensive testing

Architecture:

• NeighboringOptimizationTransformer extends IrElementTransformerVoid
• Uses @OptIn(UnsafeDuringIrConstructionAPI::class) for IR access
• Integrated into AggregateFunctionTransformer pipeline
• Follows established patterns from ProjectionTransformer

Fixes #860.

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