Add deterministic R extraction

[forhappy]

What

• add a deterministic R extractor for imports, functions, assignments, containment, and calls
• route .r files and extensionless Rscript executables through the extractor
• add an R fixture covering package imports, namespace calls, multiline functions, nested lexical functions, and <<-

Why

R files were classified as code but had no extractor, so Graphify silently produced no graph nodes for them.

Impact

R source now contributes stable graph entities and relationships without requiring an R runtime or optional parser dependency.

Checks

• 379 Graphify language tests passed
• graphify update . completed successfully
• git diff --check passed