refactor: rename chain_dag to chain_let for semantic clarity

[lmeyerov]

Summary

Renames chain_dag to chain_let throughout the codebase for semantic consistency with Let bindings.

Stack Position: #4 - Builds on the full GFQL implementation and provides semantic consistency.

Stack Order

The complete GFQL stack (merge in this order):

1. PR feat(gfql): PR 1.2 - Basic Working DAG execution #706: Core AST classes (ASTLet, ASTRemoteGraph, ASTChainRef)
2. PR feat(gfql): PR 1.3 - Call Operations for safe method execution #707: ASTCall implementation with validation
3. PR docs(gfql): Consolidate wire protocol documentation #709: GFQL directory restructuring and import path updates
4. PR refactor: rename chain_dag to chain_let for semantic clarity #713: This PR - Rename chain_dag → chain_let for semantic consistency
5. PR docs(gfql): Comprehensive Let bindings and Call operations documentation #708: Comprehensive documentation (rebased onto refactor: rename chain_dag to chain_let for semantic clarity #713)

Semantic Rename: chain_dag → chain_let

Rationale

The original name "chain_dag" was misleading because:

• The implementation executes DAG (Directed Acyclic Graph) operations
• "Chain" implies sequential operations, but Let bindings enable parallel/branching execution
• "Let" better reflects the semantic concept of variable binding and reuse

Files Renamed

• graphistry/compute/chain_dag.py → graphistry/compute/chain_let.py
• graphistry/compute/gfql/chain_dag.py → graphistry/compute/gfql/chain_let.py

Function Renames

• chain_dag() → chain_let() - Public API function
• chain_dag_impl() → chain_let_impl() - Internal implementation
• All related helper functions and references

Import Updates

• Updated all imports from chain_dag to chain_let
• Updated function calls throughout the codebase
• Updated test imports and references
• Updated documentation references

Test File Updates

• test_chain_dag.py → test_chain_let.py
• test_chain_dag_gpu.py → test_chain_let_gpu.py
• test_chain_dag_remote_integration.py → test_chain_let_remote_integration.py
• Updated test names and docstrings to use "let" terminology

API Compatibility

• Added deprecation warnings for old chain_dag() function
• Maintains backward compatibility during transition period
• Clear migration path documented for users

Documentation Updates

• Updated README references to use chain_let terminology
• Updated integration test documentation
• Updated code comments to reflect Let binding semantics
• Enhanced clarity around DAG vs sequential operation concepts

Benefits

• Semantic Clarity: Name now matches the actual functionality
• Consistency: Aligns with Let binding terminology throughout GFQL
• User Understanding: Clearer mental model for DAG-based operations
• Future Extensibility: Better foundation for advanced Let binding features

This rename completes the semantic alignment of the GFQL implementation, making the codebase more intuitive and maintainable.

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[lmeyerov]

Superseded by PR #715 which consolidates all GFQL code changes into a single PR for easier review and testing.