feat: add Leiden community detection (LEIDEN/LE) to ALGO extension

[chiangchenghsin-hash]

Summary

Adds Leiden community detection algorithm (CALL leiden(...)) and its alias (CALL le(...)) to the ALGO extension.

Features

• CALL leiden('projection') RETURN node, community_id — Leiden algorithm
• CALL le('projection') RETURN node, community_id — Short alias
• Same GDS pipeline as louvain (GDSFunction::initSharedState, getLogicalPlan, getPhysicalPlan)

Files

• src/include/function/leiden.h (new) — Function declaration
• src/function/leiden.cpp (new) — Implementation with LeidenGraph, MoveNodesFast, GDSResultVertexCompute pipeline
• src/main/algo_extension.cpp (edit) — Register LeidenFunction + LeidenAliasFunction
• src/function/CMakeLists.txt (edit) — Add leiden.cpp to build

Algorithm

Independent C++ implementation (no external dependencies). The MoveNodesFast local moving phase is implemented. The refinement and aggregation phases are scaffolded for future completion.

Verification

• Windows 11 x64, MSVC 19.50, LadybugDB 0.18.3
• CALL LEIDEN('G') RETURN node, community_id → returns correct community assignments
• CALL LE('G') RETURN node, community_id → alias works
• All existing ALGO functions continue to work (SCC, WCC, PR, KCORE, LOUVAIN, SF)
• 15/15 Node.js API tests pass

[adsharma]

Looking to deprecate the current algo extension. Plan is to reroute the GDS functions to icebug, which already has a much more performant implementation:

https://github.com/LadybugDB/ladybug-icebug-notebooks/blob/main/ladybug_icebug_comm_detection.ipynb