Builder ergonomics: configure LightRAG / Leiden via GraphRAGBuilder

[ronaldmannak]

Motivation

LightRAG and Leiden are configured by constructing DualRetrievalConfig, KeywordExtractorConfig, and LeidenConfig and passing them to rag.lightRAG(...). A fluent builder surface (matching the existing withChunkSize/withTopK/withApproach style) makes them discoverable and consistent.

Scope

• Add builder methods (e.g. withMergeStrategy, withKeywordLimit, withLeidenResolution, or a single withLightRAG(config:keywordConfig:) / withLeiden(config:)) that store defaults on Config.
• Have GraphRAG.lightRAG() / detectCommunities() use those stored defaults when the caller doesn't override them (while keeping the explicit per-call parameters working).
• Keep it purely additive — no behavior change when the new methods aren't used.

Files (small, low-overlap)

Sources/GraphRAG/GraphRAG/Builder.swift, Sources/GraphRAG/GraphRAG/Config.swift. Small; only coordinate with the "LLM community summaries" issue if that one also adds a Config flag.

Acceptance criteria

• New builder methods set the defaults; lightRAG()/detectCommunities() honor them; existing call sites unchanged.
• Unit tests for the new builder methods.
• CI green.

Context: follow-up to PRs #2 / #3. Good "starter" / parallelizable task.