exp-algo: add Triangle Count + Clustering Coefficient

[Copilot]

Adds per-node triangle counting and local clustering coefficient as experimental UDFs under flex.exp.*.

API

• exp.triangleCount(neighbors, adjacencyMap) — counts triangles a node participates in by checking which pairs of its neighbors are mutually connected. O(d × d_max) per node.
• exp.clusteringCoefficient(triangles, degree) — computes triangles / C(degree, 2). O(1), designed to compose with triangleCount.

Usage

MATCH (n)--(m)
WITH n, collect(DISTINCT toString(id(m))) AS nbs
WITH collect([toString(id(n)), nbs]) AS pairs
WITH flex.map.fromPairs(pairs) AS adj
UNWIND keys(adj) AS nodeId
WITH nodeId, adj[nodeId] AS neighbors, adj
WITH nodeId, neighbors,
     flex.exp.triangleCount(neighbors, adj) AS tc,
     size(neighbors) AS degree
RETURN nodeId, tc, flex.exp.clusteringCoefficient(tc, degree) AS cc
ORDER BY cc DESC

Changes

• src/exp/triangleCount.js, src/exp/clusteringCoefficient.js — implementations
• tests/exp/ — 13 integration + unit tests covering triangles, K4, star graphs, numeric IDs, partial adjacency maps, null/invalid inputs
• docs/exp/ — per-function docs with syntax, examples, cost notes
• docs/README.md — new Experimental Algorithms section

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Original prompt

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This section details on the original issue you should resolve

<issue_title>exp-algo: add Triangle Count + Clustering Coefficient</issue_title>
<issue_description>## Summary
Add experimental support for triangle counting and clustering coefficient metrics (local and/or global) under flex.exp.*.

Why

These metrics complement community detection and centrality work already under discussion, and they are not part of the currently documented FalkorDB built-ins.

Use cases

• Fraud / sybil ring discovery
• Social cohesion and trust analysis
• Community-quality validation
• Graph quality checks and local-density analysis

Suggested scope

• Consider paired helpers such as triangleCount and clusteringCoefficient
• Clarify whether outputs are per-node, graph-wide, or both
• Add docs and integration tests
• Note expected cost on dense graphs
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes exp-algo: add Triangle Count + Clustering Coefficient #101

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